The basics that I was given when I started to play, and which I use today, include:
Try to stay within your trained schools as much as possible. That being said, there are some spells that most Mages will find handy (I usually pack at least one of many of the ones mentioned by Maverick). You may have to go out of school for some of those. For example, a Wizard generally chooses between two torso armor types... Dragonscale Hauberk and Storm Drake Hide. My Wizard will either choose one of these, or pack one each. The Wizard isn't trained in either school related to these spells, however.
Similarly, take a spell in an opposed school only with very good reason... but don't reject a spell just because of this. Once in a while, for example, I find that I put a Battle Forge in a Beastmaster book even though it costs five spellbook points to do so.
Try to limit your Creatures to 1/4 of your book in terms of spellbook points and cards. So... starting with 120 spellbook points, my personal maximum is 30. This is enough to give you the ability to have a variety of Creatures (again, most in trained schools), and helps to ensure that you will get to play many of the ones that you include during the game.
If you need to cut back on spellbook points, look at spells that you have duplicated. Do you really need a fifth or sixth Thunderift Falcon? You'll figure out where you have too many of a particular spell as you get to play. If there are cards that you never seem to cast, then you pretty much know which ones may be more expendable than others.
In terms of equipment, remember that you can not have two spells taking up the same equipment slot at the same time. Sometimes this is ok, but care should be taken that you are not wasting spellbook points this way. The example provided earlier is of having two pieces of torso armor. There are other reasonable valid duplications (three staves / wands, for example, figuring that what you need may depend on the matchup; or two identical items that is a key for the construction of the book (Johktari Beastmaster Hunting Bow, Warlock Lash of Hellfire, etc.)
In general, having a balance in spell types and spells, as well as being able to play the book using a second strategy is preferable to having the "perfect" book that works only against some Mages some of the time.
I hope this helps!