House Rules of Magick:

1. Doing rotes is way easy.
Rotes that you've taken the time to learn are practically second-nature to your character. You've invested the practice in them so that when you flick your wand and say 'Expelliarmus!', you don't have those embarrassing backfires. As such, getting simple success on a coincidental rote doesn't require a dice roll, even in combat, and even for vulgar rotes, you can't botch. For any effect that requires more than a single success, like damage, duration, or modifier, you should probably make the roll.
Note that you don't have to pay XP to learn Rotes in M20. You do have to spend a few hours practicing the rote (more for higher-rating rotes) and you do have to make an Intelligence check using either the appropriate Esoterica (for magickal spells) or Science (for enlightened procedures). Obviously, your Foci have to match those spelled out by the Rote, or you can't learn it. There are no limits on the number of rotes a character can learn, within time constraints. However, if anybody abuses that freedom, we'll houserule it. I don't know about you, but I don't want to have to keep up with 5 pages of rotes for a single character.

2. Unless you're under pressure, you can always do awesome magick.
Simple success on simple effects for which you have the Spheres shouldn't need a dice roll unless you're resisted or working under pressure. If the degree of your success matters, though, you'd better roll for it.

3. Magick isn't always something you do.
You're a Mage, not a Sorcerer. Magick isn't what you do so much as what you are. As such, you may experience or exhibit magickal effects, even when you aren't actively casting them. Such effects are entirely under the purview of the Storyteller, and you can't depend on them--that's why Mages invented spells, after all. But, especially for one-dot perception effects, don't be surprised if your character randomly exhibits magickal ability within the realm of his or her Spheres that he wouldn't normally possess except under the effects of a formal spell.
These effects are spontaneous, but they're most often triggered by investment or necessity. If the character sees a malicious bully prodding a defenseless snake at the zoo, he's more likely to respond by making the glass disappear at that time than if he's leisurely studying the snake under the lamps.

4. Your Affinity Sphere says more about you than which Sphere is cheapest to advance.
Your Affinity Sphere is more about what kind of magick resonates with your avatar and less about who your teacher was. As such, in this setting, you can pick any affinity sphere you like, regardless of your background.
In addition, the aforementioned spontaneous magickal effects are much more pronounced within your Affinity Sphere. A Chronomancer with a Time Sphere affinity, for example, is more likely to catch stray glimpses of the past or future; while an Evoker with a Forces affinity might spontaneously ward off the occasional bullet or knife thrust.
Again, although these effects are more pronounced, they're still not dependable. These may be awakened glimpses of the true nature of reality from your avatar's perspective, but they're only glimpses.

5. You can learn Rotes you can't technically cast.
You can learn a Rote with a Sphere requirement one dot higher than you actually possess as long as you have at least one dot in each required sphere--that's what practice does for you; although, expect it to take longer to learn a Rote for which you don't have a required sphere rating. You can cast these effects as if you had the required spheres, but you can't cast them automatically the way you can cast Rotes within your abilities. These, you have to roll every time.

6. Prepare beforehand to exceed your limits.
If you play into your foci, you can fudge your Sphere levels a little when you need to. This probably will cost a point of Willpower, which also means you won't be able to spend Willpower for an automatic success. However, a word of caution for the ambitious: Don't Botch. Expect extra Paradox if you're attempting to cast a spell above your ability.
If you draw circles, playing into your focus means using orichalcum dust to draw your circle. If you burn candles, it means pulling out the black candle with the wick woven by three Grand Masters. If you use mathematical formulae, it means taking the time to permute the variables through the golden ratio tables, or running them through the theoretical computer algorithm, which you prepared for a situation exactly like this one.
Playing into your focus means taking the time beforehand to stack all the odds in your favor when you really need them. A prepared wizard always keeps an ace up his sleeve when he knows he may need it. If you do the same, your foci will not fail to come through for you when you need them the most.

7. We're using the Allocating Successes rules (detailed on M20 p. 538) for spellcasting from Second Edition, Revised.
You can use extra successes in this system to buy damage, duration, and additional targets, according to the Optional Dividing Successes Rule chart on M20 p. 504. It gives you a little more versatility in spellcasting.

8. Your Focus can provide secondary skills for Arete rolls.
Starting mages have crap pools for making magick--everybody knows that. You're sweating just to get that Night Vision spell to work without botching, much less fireballing anybody. However, if you're minding your Focus, it can provide you with secondary skills that can reduce the target number of Arete checks, making them easier to hit, even with fewer dice.

9. Most Arete Rolls can turn into Extended rolls.
You don't actually have to be performing some kind of mystical ritual to make an extended roll for spellcasting--in fact, in this campaign, you can turn any spellcasting roll into an extended roll if you need a few more successes before you cast. Just declare that you're spending another turn casting the spell. You can release it at any point, with three caveats:
  a. If you roll a failure, you must release the spell with the successes you've accrued to this point, or else it collapses. This includes failure due to Countermagick (as described in M20 p. 545), and Countermagick can counteract successes you've already banked. Obviously, this makes Countermagick a serious consideration in making extended spellcasting rolls.
  b. If you roll a botch at any point, no matter how many successes you have in the bank, the entire spell goes sideways and you suffer Paradox for it. Even worse, if you suffer a Paradox Backlash and the spell is intended to deal damage, the successes you've already accumulated add to the damage for the Backlash.
  c. If somebody wounds you while you're in the middle of extended casting, you may have to make a Stamina + Meditation check to keep control of the spell. The Difficulty for this check depends on the type of damage (Bashing: 6, Lethal: 8, Aggravated: 9) and the Threshold is equal to the number of Health Levels taken. If you fail, the spell simply fails.

10. A mage doesn't actually have to start with Seven Foci.
A mage's Focus is a very personal reflection of his or her Art, and as such, sticking a number to it--even a mystically significant one--trivializes the importance of the focus in the mage's worldview. Besides that, some categories of focus are much broader than others, and it's difficult to compare the statistical worth of wands versus computer programs.
As such, a starting mage needs some foci with which to work her magick. Whether it's three foci or seven foci or nine foci is up to the player.
This also means that mages don't automatically lose access to their foci as they improve in Arete. If a mage wishes to give up her foci, that's something better handled through practicing using magic without foci and through resolving personal goals and quests than through an arbitrary system mechanic. If a mage prefers to keep using foci, that's fine, too.