Some things that I can think of (and are written down above me also):
- Grouping spells: Putting similar spells from different character on the same button. Group everything that can be grouped so you have the least keybinds as possible. This is very important.
- Using modifiers to do alternative casts, like ALT for doing AoE or Shift for DOT(curse, flame shock, poisoned shot). So besides grouping a specific combat action(healing/dps/buff) you can make differentations like single target vs. AoE or single hit vs. DoT, etc.
- Use ESDF instead of WSAD so you have more surrounding keys around your movement keys. This allows you to use more keybinds that are easily available (so you have to move your hand less).
- Avoid clicking, using the keyboard is MUCH faster. Clicking is only okay for very rare occasions like resurrecting/summoning people.
- Use castsequences for specific classes that have a certain rotation for casts. If you have many mana-based DPS'ers, then you have to consider the mana consumption of those sequence casts versus their total mana, because you don't want 1 of them to have deplated his mana while the others are still at 50%.
- Use castrandom for classes that have spells on cooldown: castrandom will ignore all spells that are on cooldown until they are available. E.g. "/castrandom lightning bolt, frost shock, frost shock, frost shock, frost shock, frost shock" will almost always cast a shock whenever it can, but will only use lightning bolts when the frost shock is on cooldown
- Consider your keyboard layout VERY carefully. I have mine over here: http://farm3.static.flickr.com/2138/...bb107ab2_o.png (although I use more keys then are marked)
- Weigh the pros and cons of your setup: do you want 1 button(with modifiers?) for healing which allows you a simple setup yet you have to select a target to heal each time you want to (or have your targettarget healed) OR have 5 buttons for healing where each button will heal a specific character.

Most of it is personal preference of course.