Rinse it in some peroxide and water.

If you just need to clean it a little, you could brush it with a toothbrush and some toothpaste. you could also soak it in mouthwash or peroxide.

The cheapest technique to do it's with a pot of boiling water... That isnt always harmless depending on what kind of material your retainer is made out of, so i wouldnt advocate it. Another thing that works really well for me is something called Efferdent. You can buy it at the grocery store, and it works very well. You fill a cup up with hot water, put the tablet in, then leave the retainer in it for a little while. It is denture cleaner, but it has multi purpose. Another technique to do it's with your tooth brush and tooth paste. That is the easiest technique to obtain it done.

Since your mouth is not sterile, there is no reason to sterilize your retainer. Just wash it well with soap and water. If you want to do something extra, soak it for a few minutes in Listerine, then let it air dry. As the alcohol dries, it kills germs.

Whatever you do, don't boil it!! The best thing to use is Efferdent (orgeneric) denture cleaner. Soak it in warm water and one of those tablets. Let it soak for an hour or two. Then it would be all clean.

Don't boil it like some of them are saying. It could warp the retainer and then you can't wear it. =)