and be prepared as best you can.

They key is to make sure people are having fun, and you'll need to adapt on the fly frequently to keep this element.

I couldn't tell you how many times during the night I adjust the number of enemies, or adjust HP's, or adjust tactics, all to keep the action lively. It could be that what I planned turns out pathetically weak...and needs some bolstering with more numbers, more HP's or new powers, etc. It could also be that what was planned turned out way too strong for the party, and so scaling back certain aspects avoids a catastrophe. Though I normally comment on the parties impending doom (basically every session) I don't really want them to get totally wiped out. It's happened before, but I try to avoid it for the most part.

Nothing is set in stone, basically, and you control what happens outside of player decisions. Also, find ways to give hints when needed, or provide information you think will help the group during deliberations. It's one of the best parts of being DM, I think, to sit and listen to the group plan out strategy based on usually faulty information. I try to hide my smile. Sometimes they nail it however and this is also pretty cool. But other times they are way off, and basically are just fishing for anything, or leading themselves to disaster. This can happen outside of a fight, or during a fight. I use Perception checks, Insight checks, etc to provide more info to characters. This is driven by what they are doing in-play, so as to avoid the deus-ex as much as possible, and I try to make it subtle....not "player-Y, roll a Perception." "22". "you turn around, walk back down the hallway, bend over at the waist on your 25th step, and notice a latch near the floor." Instead, (I would have had them roll perceptions at the time they went past the latch, and assuming they all failed), I would take who was the highest roller and allow them an insight-check, and then say something like "you can't be sure, but you recall a flash of what was seemingly a metalic object while moving through the halls, just caught out of the corner of your eye, and standing out against the wood/stone". It would then be up to the party to work this out, go back and inspect areas, and ultimately find the latch themselves.

Another fun thing I do is have the party make rolls when it's completely unnecessary! This really throws them off, as they think something important should have just ocurred, but they missed it. It may delay some time as they blindly search for what they "missed", but it keeps them on their toes and removes the idea that every time a roll is made something needs to be happening. It keeps things a little more exciting IMO.

Overall, remember to plan, have fun and be flexible.
Good luck!