What kind of speakers/sub set do you have?
If it's a computer speaker set, the crossover is probably high enough that the sounds coming from the sub will be quite directional and you'll want to keep it in a central location between your two speakers on the floor somewhere.
If it's more of a "real" speaker/sub combo, the sounds coming from the sub will probably be difficult to localize and you can get away with more placement options.
In the second case, one quick thing you can do is to place the sub at the listening position playing some music with some decent bass in it. Crawl around the room to various potential subwoofer locations and decide where things sound the best. If the trick works, when you place the subwoofer at that potential location, things should sound good back at the listening position.
Often a location in the corner of the room is a good starting point.
You can get much fancier with getting things set up if you want to.