Build your own speakers!
Obviously this is my number one tip. It's good fun to do and very pleasing when they work well! Instead of paying top whack for brand new drivers, consider looking round the electronics surplus companies. Or what about going to the scrapyard and getting some old car speakers?

Improve the speakers you've already got
Take the grille off. If it's cloth stuck straight onto the cabinet, cut it out round the driver openings very carefully with a craft knife. If you still want to protect the drivers, cover them with wire mesh (1cm square stuff meant for Rabbit hutches from B&Q is good). Change the crossover capacitors for higher quality types. Upgrade the wiring to decent quality cable. Change crappy spring clips for proper 4mm binding posts. Add damping, as proper acoustic wadding or old woolly jumpers (listen to the effect - bass and midrange should improve). Brace the cabinet (glue in 12mm dowel rods) if it's thinly built. If the drivers are in good condition but the cabinet's a mess, copy its internal dimensions, driver layout and port and build yourself another, preferably using thick MDF (paint it as wildly as you like). Build a compartment onto the bottom, with access from the back, so you can mass load the speaker (use sand, pebbles or a couple of bricks). If the speakers are single way units with poor treble, increase treble output by adding a piezo disc to the front, wired across the main speaker terminals (drill a hole to put the wires through and glue it in place over the hole).

Buy some stuff secondhand
If you haven't got something in your system, then buy it cheaply secondhand - it's better to have a crap tape deck (say) than none at all. If it's something with a mechanical mechanism (like a CD player, tape deck or VCR) that will eventually wear out then don't spend too much money, as it may well be worn out already. Amps, tuners, loudspeakers and turntables are usually good value secondhand. On the other hand, if you find an old system for only 10 pounds round the boot fair, what are you complaining about? If when you get it home it doesn't work, you've then got the fun of fiddling about with it and fixing it. Remember to look in the charity shops, small ads, secondhand shops, and cash generator type places as well as at boot fairs.

Donated equipment
Blag stuff! Parents and relatives will often give away their old equipment. Make it known amoungst your family that you like tinkering with old audio things...

Make your own cables
When you get a hi-fi dealer to make up cables for you, all that's happened is his trainee has got the soldering iron out and soldered the connectors on. Cables in a pack? Someone else soldered them at the factory! Learn to use a soldering iron, make your own, save a packet. Get your parts either from an electronics shop or your hi-fi dealer. In my experience the dealer will have better speaker cable, but will not stock unmade signal cable. If you want better quality signal cable than available at the electronics store, then buy a huge long interconnect from your dealer, and cut it up and fit your own plugs!

Cheap surround speakers
Dolby Pro Logic surround only needs a frequency response of 100Hz-7kHz for the rear channel. Almost any speaker system will cover this frequency range, and thus when you're starting out, you can use the cheapest speakers you can lay your hands on for your rear surrounds. I use the speakers from my parents old midi system. Don't skimp on the cable though - you're likely to need a long length of it, and 14 metres of bell wire has a hefty resistance! Remember some cheaper surround amps (including the one I have) use a mono amp with the speakers wired in series for the rear channel - this makes half decent cable even more important.

Make your own isolation shelves
What is an isolation shelf? The cheaper ones are just a big block of wood with feet on each corner. Get a piece of thick MDF and paint it (car spray paints are good). Put a squishy rubber foot on each corner (get from your electronics store). If you've got something that gets hot (like your amp) and want to isolate the rest of your equipment from its heat, then fix the feet on the end of long bolts or get tall feet. Stick a piece of ordinary kitchen tin foil to the bottom (use epoxy resin glue) and stand on top of your hot equipment.

If you want one of those flashy air suspended shelves, make a heavy box from MDF. Get an old bike inner tube, or a car tube from the scrapyard/tyre dealer if your box is that big. Place the inner tube inside the inverted box, and pump it up. Instant isolation!

Horrible boomy wooden floor?
If your system is placed on a wooden floor, and perhaps the people below you don't like listening to your music, or you don't like the boomy blurring it adds to your bass, then you need to place your speaker stands (and subwoofer) on something heavy that won't move too easily. Big blocks of polished granite are pretty but not good for your bank balance. Get some paving slabs! Cheapo concrete garden ones are OK for small speakers - wash them and then seal them with spray car laquer to stop your room being covered with concrete dust. You'll need something bigger for a large subwoofer - get one of those thick concrete paving slabs the council uses from an architectural salvage yard for the same price as a little one at your DIY store!

Make your own switchbox
Not got enough inputs on your amplifier for all those additional low quality sources you've acquired? Get a 2 pole 6 way rotary switch, 7 pairs of phono sockets, a box and some cable from your electronics store. Make yourself a switchbox and connect your PC, Playstation, walkman, second tape deck, TV and VCR all through your AUX input.

Build your own music racks
Furnishing store music racks are posh but expensive. The plastic ones you see in record shops are cheaper, but flimsy and still expensive if you've got a lot of CDs, tapes, records, videos or whatever. Get some 12mm MDF and make a frame with shelves 15cm apart and 14.5cm deep for CDs standing on their side (you need 15cm height so you can get your fingers in to get the CDs out!). Make the frame rigid by gluing a 6mm MDF sheet to the back. I made shelves that can hold nearly 1500 CD albums for only 20 pounds...

Read books
Read books on hi-fi, electronics, speaker design etc. Simple enough - but expensive if you buy them yourself. Then don't! Get onto your local library. If they can't get them and you pester them enough, they then should buy them for you. As I said on my speaker design page, get hold of The Loudspeaker Design Cookbook by Vance Dickason, ISBN 1-882580-10-9.

Borrow measurement facilities
Measurement facilities and test equipment are expensive. Some things are totally impractical for the home constructor to buy, even if they could justify their purchase. So ask nicely and try to borrow other peoples equipment. Your school, university or work may well have an oscilloscope for instance - and if you can't take it off the premises, bring your project to the equipment during your lunch hour. If you work in an office, surely you know someone in the family or a friend who works in engineering?

Over-capacitor your amplifier
Got a cheap amp with not much power, not much bass and that clips horribly? So long as you don't run it at or near full power all the time, and it doesn't get hot driving the speakers you have, you can squeeze more out of the amplifier by increasing the power supply capacitance. I've done this on both my Rotel RA-920AX amp (added nearly double what it originally contained) and portable Sanyo music box, with similar results in each case - deeper, more controlled bass, and greater output before clipping. Don't over do it though - watch for hot/humming transformers and hot bridge rectifiers (I had to heatsink the Rotel's bridge). I never blew any fuses, not even when running the Rotel into 3 ohm speakers.

Squeeze the last ounce of performance from your cheapo system
My first audio object was a square mono tape recorder. I wired the amplifer to an external speaker to get more volume and bass, and took the internal microphone out on a flying lead to to isolate it from the noise of the tape mechanism. By the way, it's not recommended to wire up both speakers at the same time - the amplifier won't like the extra load. If you want more than one speaker, get hold of two 4 ohm speakers and wire them externally in series to make 8 ohms.
For many years I had a Sanyo stereo twin tape portable system which underwent many modifications. The amplifer was over-capacitored as above. The amplifier output capacitor (the one between one of the speaker wires and the amplifier) was increased in value by approximately 4 times - this makes the cutoff frequency of the amplifier lower, i.e. increases its bass output. Power supply and speaker wiring was replaced with thicker cable. Damping was added to help cancel the rear radiation from the speakers - cut up sections of old jumpers were placed over the back of the drivers. In my system the radio PCB was separate, connected by four wires (+V, OV, left and right). It was a simple job to wire in a stereo jack socket so I had a line input, into which I plugged my walkman when the tape decks wore out. To power the walkman a power socket was added and wired to the power supply, and a voltage regulator was built into the cable. The unit could be powered from 6 D batteries. I considered buying nicads, but these would have the same voltage as the 6 cell battery packs I had already for my radio-controlled car, so I simply wired in an RC plug in the battery compartment (and glued in a piece of foam to hold the pack still). By far the best modification however was adding external speakers: