With completely stock seat belt and ///M Sport seats, I'm able to completely lock myself into my seat so I can't move anything but my arms, legs, and head. It's great for stability on a tight track.
What I do (requires power seats):
- Move the seat well back from your normal driving position.
- Latch your seat belt.
- Pull all the slack out of the lap belt, and as tight accross your lap and chest as you can and feed all slack into the tension mechanism (so your hand with the belt in it ends up near your outside shoulder...don't let go).
- Now jerk out/down so the retention mechanism latches and hold pressure against it so it stays latched.
- Using a free hand, hit the power forward button on your seat and move it to your track driving position.
If you did it "right", the belt should now be approaching uncomfortably tight with you held against the seat. There should not be enough play anywhere such that the belt could unlock itself.
This technique works wonders for me.