The best advice a DM will ever hear:
Improvise!
I can fondly remember a session of The Alliance(My home-brew Superhero game) in which I introduced a guy on death row to the party. When they met him they didn't know who he was, but they saved him from an explosion. Once they found out who he was, his purpose in the adventure was to stand between them and a monster so the party could escape. One of my players decided he couldn't allow that; even a serial killer didn't deserve to die at the hands of an alien monster.
The party had met at a gas station earlier in the session so the player asked me "Would it be acceptable if I had bought a bottle of water while I was there?" I said sure. The monster chasing them was on fire. He said "I take some of my extra bow string and tie the water bottle to one of my arrows." Green Arrow wannabe looks at me expectantly. "Will this even work?"
Rookie DM moment. I froze. Nothing in the rules covered water bottles tied to arrows, or spraying water on this particular monster. I hadn't specifically given the monster a weakness to water, but I could see the logic in it slowing him down. I shrugged my shoulders "Roll to see if you hit." As he totaled up his numbers, I panic planned. What do I do if it hits? What do I do if it misses? I obviously didn't want one of the party members to die just yet, but I couldn't let him truly damage this thing until the time was right.
"16?" He looks to me. I check the stat block. "Hit." Improvise mode engaged. "As the arrow hits, the plastic bottle melts and water sprays across his face. He stumbles back. You should probably run before he recovers."
I also remember him saving the convict 2 more times in the same session. He wouldn't let me kill him. Which leads to another tip. Even if he's meant to die in one encounter, give all friendly NPCs at least a simple background and personality. Because your players are going to do everything in their power to screw you over.
A truly harsh DM might have let the hero die right there. In fact had it been a Dark Fantasy I might have. But it was a superhero game, and superheroes do extraordinary things all the time.
An example from the player's side. Recently in our 4e game we faced a Lich. I was planning to take the Ritual Caster Feat and when we beat the Lich an idea struck. Lich Dust could be used for some powerful rituals if the DM is willing to use his imagination. So I scooped up a handful of the dust and put in one of my empty coin purses. The DM looked at me like I had 2 heads. What did I think I could do with it? I respectfully explained that I had an idea I would tell him later and maybe having the dust would mean the Lich couldn't raise itself or would raise with a piece missing. I wasn't sure how Lich raising worked so why not give it a shot.
I also happen to be a bit of a backstory whore. I can't make a character without coming up with how his grandparents met and why he doesn't like the taste of fish. The backstory I had come up with for my 4e character left me with a lot of things I threw at the DM for possible use in the campaign. I don't expect him to use it, but I love when my players give me a way to incorporate their characters into the campaign. So its there if he needs it, and I'll play as if it was happening if he doesn't.
For instance Damasei(My Tiefling Swordmage) serves a Devil. An intelligent Pit Fiend named Medja. He didn't make a pact with this Devil, he was forced into it by a pact his father had made. Damasei learned of the pact when he was a teen and has been using the power he recieved ever since. He aims to destroy the Devil and his contract someday. Medja, however, knows and expects this. So he sent a lesser Devil to keep watch over the Tiefling and make sure he continued to follow orders. This lesser Devil comes in the form of my Arcane Familiar. And now that I've hit level 4 I've gained a new Feat in which I transform into a creature of evil similar to a Devil for one encounter per day. I asked the DM if I could absorb the lesser Devil into me in order to activate the transformation. It sounded cool, and I found I don't really like keeping track of my Familiar in the heat of battle as much as having him around during conversations. So absorbing him keeps him out of my mind for the fight.
DM incorporated the absorbing thing a couple sessions ago, before I even hit level 4. I nearly jumped for joy.
Improvisation is fun, and neccessary, for the DM to be good at what he does. It also makes the game feel more alive.
