The Rule of Five: Wearing Several Magic Items
The Rule of Five states that one person cannot use more than five magical items at a time. However, any one item can contain up to five spells. This means that a person can have up to 25 spells going for him at once -- which ought to be enough.
Of course that won't be enough for the Power Gamer, but they'll just have to lump it. And in practice the average magical item will have far fewer than five spells in it. That could make for some interesting choices for the players as they try to decide between this magic wand and that magic ring. On the other hand, you might also want some special rules to take into account artifacts and other legendary magical items of great power. If someone finds the Ring of Gaxx, which has nine different powers, it might have to count as two items.
You could work any number of variations on this rule. For example, in a more gritty game you could have a Rule of Seven stating that a character can have no more than seven spells - from magic items or elsewhere - going for them at once. That could help keep wizardry from overpowering the game and really annoy the Power Gamers.