1- forget about controlling camera zoom in and out using your mouse wheel, bind those somewhere else.
2- bind mouse wheel up and mouse wheel down to any of your spells.
3- use shift, alt and ctrl combined with the mouse wheel up and down to get an additional 6 binds bringing the total to 8.
4- (for healers mostly) key bind macros that cast a different spell depending on whether your target is friendly or foe, an example of this kind of macro is
/cast [help] Renew; [harm] Shadow Word: Pain
That marcro ^ will cast shadow word pain on foes but renew on friendly's. Using this method you could bind 16 spells to your mouse wheel alone.