Like Byte56 suggested, limiting how many of a given type of one-shot items a player can carry pushes players towards using them. Another possible push is to make the game so difficult without one-shots that players are practically forced to use them. This of course also means that you have to make them reasonably available and provide some means for the player to acquire more if they run out at a critical point, this could be by letting them run from the fight to go back and buy or farm for more, or by dropping such items for pick up during battles, so that all the player has to do is dodge until enough items have been dropped.
Think of ammo in shooters or health potions in RPGs, both are pretty much mandatory to use, but they don’t cause any problems because they are supplied in surplus. Though they are perhaps not as exciting as a rare one shot item.
A different take on the matter is to provide non-consumed items with long cooldowns instead, like 15 minutes. Effectively that is maximum once per battle, and only for use in some battles if the player want to go at a reasonable pace. But the player always know that if the effect is really badly needed in some later battle they can just wait for the cooldown before entering the fight.