Hmm, players would probally expect, that any oppostion guarding the entrance is either just inside the cave, or waiting outside. So try fooling with their expecations.
Have some archers hidden in the area above the cave if possiable. Nastier things can be done if the enemies have enougth warning.
For example some of the enemies could cast invisbility spells on themselves (leaving only a few token guards visible so not to worry the party to much), and wait until the party passed them and attack from behind, when the archers I suggested earlier start firing on the party, and some more enemys come from out of the cave mouth. But this obivously requires warning in advance for the preparations.
A statigicly placed pit trap along the direct approch can cause a surprise, as rarely do players think about traps when in combat, so when the big strong fighter falls down, suddenly the party is without some of their frontline, and the time it takes to get out of the pit, is suddenly a lot more significant as the partys enemies are still attacking.
Thats all I can think of for now.
