Stereophile magazine editor John Atkinson's recent article, "Getting the best from your loudspeakers," provides gobs of useful information.
Atkinson's observation that "inexpensive speakers, optimally set up, may well outperform more expensive models just plonked down willy-nilly" is absolutely true. Proper speaker placement is crucial to getting the best sound out of your speakers.
The article explains how room acoustics affect the sound of speakers, and it's definitely a complex relationship. You won't need a microphone or any technical skill to get the job done, but an Atkinson-produced Stereophile test CD can be a big help when fine-tuning speaker placement.
Perhaps his most important bit of advice comes early on: "Entire books have been written about the relationship between loudspeakers and room acoustics, but the starting point for any successful setup is to position the two speakers and your listening chair as the pieces of an equilateral triangle; that is, the speakers are each as far away from you as they are from each other."
Check with your owners manual, but it's usually a safe bet to have your speakers' tweeters positioned at your ears' height when you're sitting down. Also try to avoid placing speakers in, or within 2 feet of a corner. Unless your speakers are designed for wall mounting, they will probably sound best a foot or more from the wall. Atkinson provides far more detailed info on that score.
Atkinson's feature looks at stereo speakers; I'll tackle 5.1 channel placement soon.