Using frequent flyer miles can be tricky, and sometimes it just doesn't work, but there are things you can do to improve you chances.
Be flexible:
Being flexible is the best way to improve your chances of successfully redeeming frequent flyer miles. If you can switch your dates even by a couple days it can be the difference between a reward ticket at 25,000 miles and one at 50,000 miles, or maybe even not finding a ticket at all. Flexibility with cities can also make a big difference. For example, there might be a mileage reward going to Fort Lauderdale but not Miami, and the two are close enough that it might be worth the extra driving time.
Check multiple sites:
Remember to check out the sites of the partners of your frequent flyer program. I once redeemed American Airlines miles to go to Barcelona, but the flight was on Iberia and the flight did not show up immediately on search through the American Airlines website.
Use a third party resource:
While you can find information on your own, some people prefer to pay a little extra and have a tool that combines multiple searches. If you are looking for an easier way to search for frequent flier mile rewards or upgrade, then you may be interested in ExpertFlyer.