Coredump said:
Another tactic is to have the numbers 'trickle' in. More creatures come in round 3. Or somesuch. That way they get the larger/harder encounter, without getting overwhelmed.
Plus, it tends to add a sense of tension, since encounters are no longer "one at a time".
This trickle-in tactic is one of my personal favorites, if done welll it can really allow a dm to tailor the difficulty of encounters. If you're running pre-written adventures (published or self written) and don't want to add alot to the total experience its important to be aware of the populations of area's nearby the current encounter. These 'next encounters' are your extra oomph for the current encounter.
The typical bar fight is a pretty loud affair, now add in the clash of metal weapons and armor, a few death screams, maybe a lightning bolt or two. It's very likely that 'the encounter in the next room over (or up the hall)' is going to hear whats going on.
Now, if I'm orc warrior bob, and I hear a bunch of my compadre's doing some butt whipping I'm pretty likely to want to go join the fray.. if it sounds like my buddies are on the recieving end of the butt whooping I might run and warn anyone not already involved.
So, maybe the pc's have an easy first battle in the dungeon.. but come fight no. 2 the orc warriors are ready and waiting, or even actively hunting the party.
Another useful, but definately not to be overused trick is sleep interruption. RAW says spellcasters must have 8hrs of sleep to regain their spent spell slots. Sooooo.. send a couple of sling equipped stealth goblins (or whatever) to whack a few sleeping pc's. Won't hurt them much.. but it can wake them up. I wouldn't recommend specifically targeting spellcasters though, use a random roll, unless you have a reason for the enemy to be able to identify them as spellcasters. I again want to caution you not to overuse this trick.. it can be really frustrating for a pc to have his best mojo repeatedly shut down.
If you can, try to have one or more of the monsters run away towards help. If the pc's just AoO, or otherwise nuke him, well that happens and after the first one gets away (and comes back with friends from another encounter) the characters'll really start to feel that taking out the runner was worth the effort.. at that point you can start to do devious things like set up ambushes, and use suicide trapspringing runners, (I love using that tactic with kobolds, just seems appropriate for the sneaky li'l twerps)
Finally as another poster said, read the monster descriptions, know every little combat trick the monster can use, and use it. Just remember, you're not out to kill players, but you want them to think they could die.
Hope all my longwinded drivel helps, sorry I'm not familar with the module yer running and can't tailor my advice more specifically.
