Good placement of speakers can make a huge difference in sound quality - and it won't even cost you any money!
Even better, you'll find a use for all that pointless geometry you learned in high school.
Here are a few general guidelines.
1. Make sure you keep speakers at least 2'-3' away from walls. With the exception of a few home-theater designs, speakers are generally intended to be used away from walls, which can act as an extension to the front of the speaker, or baffle, and affect frequency response, usually by creating a funny peak in mid-bass frequencies.
Conversely, some speakers are meant to be wall-mounted. If they are not, the reverse will happen - the much, much smaller effective baffle will reduce midbass performance, and adversely effect sound quality.
2. Make sure that the distances between you and each speaker are the same, and that the speakers are the correct distance apart. Stereo (and 5.1) sound is mixed as if the microphones were a given distance apart. If the speakers are not the same distance apart, it won't sound as good.
Needless to say, speakers should also be the same distance away from you - if they're not, the music will be out of phase, and one speaker will appear to be louder than the other. While it's possible to compensate by adjusting the balance, you're better off just making them equidistant from you.
Place speakers in an equilateral triangle for listening to music - the distance between each speaker and yourself should be equal to the distance between the speakers.
However, Dolby HT recordings are intended for the speakers to be closer together - for 5.1, the triangle is a 45-degree isosceles triangle. As a result, the distance between the speakers should be equal to 3/4 the distance between each speaker and you.
4. Toe-in is your friend. While having both speakers directly forward is common, better performance can sometimes be had by moving them so that they face inwards (towards you) a few degrees. They should'nt necessesarily be pointing directly at you, though - experiment for the best results.
5. There's an old trick for subwoofer placement that I've found works pretty well, odd as it is. It works best for down-firing subwoofers; front-firing subwoofers should generally be placed between the two front channels.
First, move the main viewing/listening chair over. Place the subwoofer where the chair once was, plug it into an audio source, and turn it on. While not my first choice of music, I've found that 50 Cent is a good choice for this, as many of his tracks have a somewhat bloated bassline that is very audiable.
Then, move around on the floor, keeping your head close to the ground, until you find a place where the bass seems to be coming from everywhere - this is the correct place for the subwoofer. Replace your head with the subwoofer, and return your head to the correct position.
If you did this right, you should have the same sense of being surrounded by bass in your listening chair as you did on the floor. Of course, this method is not foolproof, but it beats randomly shuffling your sub about the floor.
6. Adjust the crossover point.
Often, HT recievers and powered subwoofers have an adjustable crossover point. Different speakers should be crossed over to the subwoofer at different frequencies for best results, and tinkering with these two frequencies can help keep nasty bumps and dips from appearing in the frequency response.
Commonly, non-HT speakers are designed to work down to about 50hz, while average HT subwoofers are designed to work below 100hz. A bump between 50hz and 100hz may occur, which can cause a "bloated" sound.