For your less used spells, you can combine them into a macro to save cast bar space.
Choose the Red ? Icon for your macro
#showtooltip
/cast [ nomodifier:SHIFT] Fire Ward
/cast [ modifier:SHIFT] Frost Ward
This one is set up for a mages frost and fire wards, I dont use them offten but like to keep them out, but to keep them out would use up 2 spaces on my already full cast bar. So using this macro I have 1 button that constantly shows Fire Ward, untill I hold down SHIFT and then it shows Frost Ward. As always the #showtooltip text is to allow mouseover info.
Ideas for combined macros... food/drink, health pot/mana pot, self bandage/normal bandage, hunter food/pet food, mage self port/portal, land mount/flying mount