I recently ran a few posts about frequent flyer miles (here and here), and I mentioned some of the travel and miles blogs that I follow. I won't mention them much more in this blog, since that's not what this blog is about, but I just wanted to call your attention to the types of offers that can be had for very little effort.
The Frugal Travel Guy recently posted that United was offering 750 miles for people who signed up for the MyPoints program. The program basically sends you emails about different offers (so you should use your "special" address for unimportant messages...you do have one, right?), and if you decide to stay in the program, you can eventually exchange MyPoints points for United Mileage Plus points. The actual program isn't terribly amazing in my opinion, but 750 points just for signing up will give you more miles than a Denver-Omaha flight, and it's free. (The Points Guy gives a good explanation of how to effectively do this.) Plus, you can simply cancel later if you want to.
I'm just mentioning this to show that if you invest a bit of time and effort, you can easily get a free trip... or more.