Move the seat back as far as possible. The further you are from the steering wheel and dashboard, the less likely you are to hit them in a collision.
Untwist the belt before putting it on. Part of the idea is to control the force of the impact with the belt. If it's twisted, you have reduced the area that can absorb the force.
Wear both lap and shoulder belts.
Fit the lap belt low across your hips, not your belly. Keep it tight. Your hipbones are large and strong and are more likely to be able to take the strain of a collision than your midsection.
Keep the shoulder strap snug across your chest. It should cross your collarbone. Never tuck it behind your shoulder; you might as well not wear it if you're going to do that. Also, you can severely injure your neck, ribs, spleen and liver if you're in an accident.
Make sure the belts are tight. Loose belts are ineffective and will allow your body to slide forward for impact.