If you look up creating campaigns or creating adventures on the internet you will find a lot of advice on what to do with your idea to make it a playable game. What they won't tell you is how to get that idea in the first place. You need inspiration and for me that was the biggest stumbling block. Here's a few suggestions for you that might jog your creativity.
1) Talk with your players.
Why shoulder all of the work yourself. Get your friends together with a notebook or two and chat about cool game ideas. What if we run a sci-fi game? What kind of sci-fi? Let's say Star Wars with the serial numbers filed off. Okay, but then why not just play Star Wars (besides why I ranted about it a while back)? Hmm, how about there are no aliens, just a bunch of modified humans. Some are animal hybrids and others are just crazy skin dye jobs and augments and such. Okay, what would be our roles? What will we be doing? How about were all secret agents for the Empire, but the Empire isn't blatantly evil like in Star Wars. It's just too strict for most people so there are fringe terrorists groups and other anarchists out to grab a slice of power.
Get the conversation going and you'll come up with cool ideas that you know your players will want to play. I confess that I haven't had much successes with this one but that's just the nature of my group.
2) Look up weird things.
New technology or weird news stories can become an excellent source for adventure seeds.
I came across a story on the net about a group who decided to build an arcology in the deserts of Arizona. I was running a game set in the turn of the 20th century and my heroes were heading out West. What would cause someone to build an arcology back in 1897? I went with a religious doomsday cult. They were convinced by a charismatic leader that the world was going to end in 1900 so he gathered up his wealthy flock and found an abandoned Silver Mine near Tombstone.
Okay that sounds pretty harmless. How are these events going to get the heroes interested in the arcology? Things must have gone horribly wrong in the arcology. Now the wealthy parishioners are kidnapping folk from Tombstone and using them as food. The heroes are entering a town where the people are under attack by a mysterious outside force. This ought to get the heroes' attention.
Seriously. You can re-skin just about anything enough that your players won't know where the idea came from, if you do it right. I ran a Star Wars game where the heroes were young jedi tasked with destroying a planetary gun before the clone fleet arrived with reinforcements. Sound familiar? Have you ever seen The Guns of Navaronne?
Honestly they are so few actual stories in the world. What separates on story from another with exactly the same premise is the uniqueness of the characters, the uniqueness of the situation, and how the hero chooses to handle it.
Take Alien and Snakes on a Plane. The basic conflict is that the hero is in a confined space with a threat lurking in the shadows that will kill him/her and anyone else trapped with him/her. The differences are one is on a space ship, one is a plane. Sigourney Weaver's character is very different than Samuel L. Jackson's character, and the reasons for why it was all happening. Same roots but different movies.
4) Start with the Villain.
Create (or steal) a villain. For me it helps to think of who is playing the villain (Kevin Spacey or Christopher Walken, etc.) Figure out what he ultimately wants, how does he intend to get it, and how far is he willing to go (for a true villain there is no line he won't cross).
Creating a compelling villain puts you in the role of a character. Once you've hatched his evil plan, you now have the source for an adventure. I will advise this, don't get too attached to your villain because your heroes will be trying to defeat him. You can stave off his quick and untimely death by making sure he uses surrogates, lieutenants who are actively doing his bidding and leaving the real threat out of reach for a while. If it is a good villain concept, this lack of contact with the enemy will make him more aggravating and memorable to your players. Just remember that you can only pull the old switcheroo for so long before your players get apathetic toward him. A villain with constant plot protection is a real game killer.
Next I will talk more about what I actually prep.
UPDATE: Jerrod 'Savage Daddy' Gunning and the guys over at the Savage Worlds GM Hangout beat me to the subject and uploaded a podcast on Tuesday covering this same topic, Inspiration.