Why you should always tie unused input pins to GND or VCC
Why you should always tie unused input pins to GND or VCC via a resistor (not directly)
Despike your chips.
This means putting a .01 to .1 uFd ceramic capacitor from the +5 volt supply to ground right at the chip. power line spikes occur when there are sudden current changes far from the power supply, and it's amazing how much trouble this can cause. Reguarding capacitors:
Watch out for class 'X' and class 'Y' capacitors
which are designed for connection to mains voltages. These are fail safe capacitors which are self healing and must be replaced with the same type.
Replacing electrolytic caps with much higher voltage rated devices can upset circuit operation.
Electrolytics don't start behaving as proper capacitors until they reach a fraction of their rated voltage. Also look out for special low ESR (equivalent series resistance) capacitors found in switch mode power supplies.
Some circuits require capacitors capable of withstanding large current pulses.
Using the wrong sort of cap in this situation would lead to overheating and other nasty consequences.
Make good power supply and ground connections.
A skinny, daisy-chained wire-wrap connection from chip to chip is probably not going to be enough. The more robust you can make power supply and ground leads, the better
Keep your digital and analog circuitry physically separate whenever you can.
Digital switching, especially at microprocessor buss or video card speeds, can throw all sorts of noise and trash into analog or audio circuitry.
Don't use silicon sealant to mount wire-wrap sockets or to seal/insulate circuitry.
This stuff is handy and common but it is not an insulator. It will leak small currents, which may not matter in logic circuits but can wreak havoc in high-impedance analog circuits.
Make sure you can get the parts before basing a design on it.
You may find the ideal integrated circuit for your application in a data book, but it may not be in production, may be unavailable from distributors, or may be too expensive. Especially if you are creating a design you hope to produce for a while, it's wise to choose devices that are widely available and that (you hope) won't be discontinued.
NEVER plug untested circuitry into a computer backplane slot!!!
Never never never, if you love your PC. All it takes is a simple little wiring error and your motherboard and disk controllers will be toast. While IBM-PC/AT buss interfacing isn't rocket science, it's all too easy to do expensive damage to your machine. Consider other interfacing methods (parallel/serial ports, commercial control/data acquisition boards) first. There are buffer cards that transparently permit prototypes cards to be used while protecting the computer buss, but they are not cheap.
Use IC sockets on prototypes.
After you have things debugged, you might consider soldering chips directly to boards to save cost and eliminate connections that can oxidize or come loose, but during the design/testing phase you need to be able to swap chips without repeated desoldering.
Pick chip types with the correct switching speed for your design
digital logic chips having very fast edge speeds (dV/dt), such as 74S, 74AS, 74F, and, best of all, 74FACT are needed for super high frequecy, low propagation designs but can cause RF crosstalk and interferrence (especially in poorly designed or laid out cicuits) that will frustrate the most brilliant engineer. See Signal Levels@
DON'T TRUST THE INFORMATION IF THE DATA SHEET IS LABELED "Preliminary Information".
This means that the data sheet was written before the chip was actually in production, and the device may change significantly by the time it is actually released. The device performance may be different; still more important, the pinout may be completely rearranged in the final design. Preliminary data can help you choose a device but don't set your design in stone based on it
Protect against incorrect (reversed) insertion of batterys
Appling Vcc to the GND and Gnd to the Vcc will fry just about anything. Use of a fuse  (or one of the PTC thermal auto-resetting devices from Bourns or Littlefuse) and a diode is a minimum. You can take an enhancement power MOSFET and put in the negative return line from your circuit. The N-Chanel Enhancement MOSFET is used "upside down" compared to other circuits. That is to say, the Drain is connected to -ve, Source through the load to +ve, and gate to +ve. The gate is driven through a 1 Meg resistor from the positive supply terminal. A correct power connection drives the MOSFET fully ON, and everything works. Reversing the supply connections turns the MOSFET OFF, and no current flows and you're protected. Correct the power connections, and you're back to normal. Select a MOSFET with a low "R(on)" resistance, and there will be very little voltage drop.The better way to go about this is to protect reversal based on the mechanical properties of the battery. mostly this is done by a positive battery terminal that is set back in the holder so only the positive battery pole that sticks out can reach it.
Cover the window on any chip with EPROM even if you don't care about the EPROM part of the chip
PIC processors for example are sometimes affected by non-uv light entering the EPROM window and shineing on the silicon.
Don't use simulators
Simulations are doomed to succeed. Reversed biased Transisters are a good example.

Signal Levels@

Surface Mount Devices@

See also:

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