What are some of the ways that competitive swimmers can decrease the amount of friction or drag between themselves and the water though which they are swimming?

Reducing drag is usually referred to as improving your "trim" in Scuba diving. There are several ways you can try and do it.

• Pay attention to body positioning. Generally, the more horizontal you are, the more streamlined you are. Avoid the "bicycle" position.
• Use more streamlined gear. For example, consider using a backplate and wing rather than jacket style BCD. If you carry a pony, mount it rather than sling it.
• Cut down on any unnecessary gear.
• Avoid "danglies". Clip in everything, or stow it within pockets and holders.
• Focus on neutrel bouyancy. Although unrelated to trim (at least in a physical sense), divers who maintain good neutral bouyancy are less susceptible to drag and resistance.