Were I going to do it all over myself, I'd introduce the concept of 'concentration' (ala DAoC -- OMG did I just make an MMO reference?) for spells with a duration longer than 'instantaneous'. Casters got a limited amount of concentration slots, and each spell they had to maintain took up one. So instead of rush-rush-rush through the dungeon, or fight/rest/fight, it became a choice of 'Do I keep up 4 bulls strengths, or drop the one on the ranger so I can summon a monster?'
Non-trivial to get the right number of concentration slots, but do-able. Also makes the higher level mass-effect buffs more useful if they took up fewer of the concentration slots.