Do keep in mind that your hits when the preparation go off will tend to be lower than the hits of a regular mage (spellcasting averages 1/3 Spellcasting pool. Preparations average 1/9 Alchemy pool + 2/9 preparation force). So any spell that faces a resisting dice pool tends to be a lot weaker as a preparation, so I agree that boosts are a good place to focus.
Michael Chandra’s suggestions are all good. Here are few other ideas:
Increase intuition is quite a flexible one. Helps perception tests of all types, helps judge intentions, helps initiative (and doesn’t conflict with initiative boosters), and even can help with drain for some traditions.
Increase Willpower has some interesting uses. They don’t show up as frequently as Intuition, but they are sometimes items that are hard to impact in other ways. For example: increase firewall, drain resistance, justifies a repeated memory test with a higher dice pool, improves resistance to a number of opposing magics, help resist certain drugs/toxins, and with two successes can wake up someone who is knocked out (keep a teammate on their feet for a few minutes, wake up someone for questioning).
Improve Aim can be handy, depending on the team. Each success adds 10m to your ranged weapon increments, which can be a big boon to pistol users.
Levitate, Barrier, and Gecko Crawl are all still useful at low hits.
Catfall can open up certain plans that would not otherwise be available (although not knowing how much height it buys you limits just how much you can count on it).