Make sure you buy a 3" harness. I had a generic 2" wide strap harness and it would dig into your shoulders and waist. Hurt like crazy.

Then my sponsor purchased me a 3" wide M&R camlock 5 point harness and it's nice. Make sure you get pull up waist belts and pull down shoulder harness belts. Also make sure you install them correctly.

Or you can do the cheap'o way of twisting your belt before you lock it. Take the tab of the seat belt and twist it one full turn with your seat all the way pushed back. Then lock the seat belt. Tug tight to tighten it against your wait. Tug on the shoulder part until it locks, then hold the belt while you move your seat forward. Works for me since I instruct in so many different cars.