Here are some things that I do to get ready to write up an adventure.
1) I always write an outline, just like I would for any school paper or story I might write. This helps me figure out where I'm going or where I'd like to go. In doing so, sometimes I get more ideas.
2) Flip through your game's "monster" books. Sometimes, just looking at the monsters or NPCs might help you come up with an idea. My son loves to just read through them and figure out how to fit in the odd and unusual monsters into his campaign, which sometimes works and sometimes doesn't.
3) Watch movies or read books of that genre. Now, you don't want to send them on a Lord of Rings style adventure ever time you game, but what if you added in being rescued my giant eagles or something of the like which led to another aspect of another adventure? Sometimes just watching movies or reading books will perk your imagination and get you going in the right direction.
4) Talk to your players. What do they want to do? Do they want to delve in a dungeon, trudge through the wilderness looking for ruins or do they want some intrigue in the city? After all, its about you all having fun. Sometimes just going with a one off dungeon delve is a great idea!
5) Read roleplaying forums. There a tons of GMs out there waiting to share how they have dealt with stuff. Maybe they'll get you on the right track.