The most important houserule I use is that my magicians use their magic attribute to resist drain instead of willpower. There are a very important number of reasons for that :

• The most logical reason : the more powerful a magician is, the more he can easily cast spells. Otherwise a double digit initiated like Harlequin has no more advantage than an archetype to resist drain, as they, well, understand magic in the same way, I suppose ? That’s what it would mean, wrong ?,
• When a magician is initiated, he really takes advantage of its initiation, because the initiation grade adds itself to the magic attribute. Otherwise, why would magicians want to initiate more than to grade zero : the astral pool bonus is really negligible, regarding the value of an average astral pool ; (s)he theoretically gets all the powers at first initiation (though they get two powers per initiation in my games : centering and masking at grade 0, dispelling and shielding at grade 1, reflective shielding and focus blocking at grade 2, anchoring and quickening at grade 3, cleansing at grade 4) ; and initiation is REALLY expensive in karma. The only advantages would be to get better allies, get rid of geasa, and, OK, get great spell defense with shielding, that's a point,
• The more a magician is cybered, the less (s)he is in accordance to astral space, and therefore (s)he has more difficulties to resist drain than someone in harmony with astral. Otherwise, a magician is really powerful, because he has interest in being heavily cybered : (s)he gets the bonuses from his cyberware, (s)he has a low magic attribute, so the drain is more often physical, and therefore (s)he can heal self and cast more, as it is impossible to reduce mental damage, when drain is stun damage. You would say it is far more interesting for a magician to have a mental drain : if he needs power in combat, getting physical damage could kill you. But if you fall unconscious, YOU’RE DEAD because it is a little bit more difficult to dodge grenades when you’re unconscious. But with physical damage, you can heal self, and therefore be more efficient because able to cast more. Anyway, a team with its magician down has lost the combat in 90% of the cases. And what do you think the enemies will do with your unconscious body ? Each time they can, my magicians try to get a physical drain,
• What’s more, a magician with a low magic attribute is much more polyvalent because he can connect to simsense (rig or matrix) easily : so, a magician resisting with willpower : has wired reflexes, reacts first, casts a spell and heals self, so he can do it again. And with 1 at his (her) magic attribute, he can connect to the matrix, and be a rigger too, with his (her) drones to help while in the run ! Who said you need a TEAM of shadowrunners ? (when connected to the matrix or rigged, you suffer a TN+1 per point of magic attribute for every action while jacked in, see Ivy and Chrome for this rule),
• Last but not the least : aren’t you bored of magicians with this Increase Willpower +4 spell with that 45 000 ¥ spell lock which gets them 4 dice more to resist drain ?

One point you could oppose me is that it is more risked if you have a deadly wound to lose your last magic point ? Yes, what is that rule exactly ? You must make less than your magic attribute with… what ? 2D6 ? Less than one with 2D6 ? This works the same with the "focus addicted" rule in Awakenings, it is impossible to get focus addicted with 1 in magic.

If you still fear a physical drain, get initiated. You will really take advantage of it, because you have more dices to resist drain.

The point is that power foci do add to the magic pool, but do not add to magic rating to resist drain, otherwise, they would add twice their value. That’s all.