Background: One of the more frustrating things that can happen when cornering at maximum g's in an M3 is to be sliding all over the place in the factory leather seats.  This is especially true if all you have holding you in place is the stock seat belt.  There are several possible solutions to this dilemma.  You can acquire a cloth covered driver's seat.  Leather might look cool but it is a poor choice for a performance seat if you really intend on "driving" the car.

You can go one step further and install a race seat.  These are actually quite comfortable and are unparalleled in the lateral support that they offer.  The only disadvantage is a possible loss of adjustability and the fact that they are somewhat more difficult to get in and out of.

The third possible way to improve your connection to the car is through the use of a harness.  Ideally this is done in conjunction with at least a performance cloth seat or a race seat.  But even with the factory leather seats a harness helps a lot. You have two choices.  You can go with a 2" wide four point harness or you can go for a 3" wide 5 or 6 point harness.  The 2" type harnesses work well with factory type seats and can be more easily used in a car without a roll bar or cage.  But although it appears less extreme these units can still be difficult to get in and out of because the shoulder straps are attached to the lap belts.  So you sort of have to wriggle your arms through the loops as you get in the seat.

I chose to go with the 3" type racing harness.  These should really be used in 5 or 6 point format.  The belts are not designed to work in 4-point configuration, as they cannot adequately guard against submarining without the sub-belt.  Submarining is a situation where you slide forward under the lap belt during a crash.  The so called "sub-belt" is actually an anti-submarine belt, and goes down from the buckle to the floor in between your legs.  These are sometimes called "crotch-belts" for obvious reasons.  The only difference between a 5-point and a 6-point harness is the mounting of the sub-belt.  The sub-belt either has one strap and mounting point or two straps and two mounting points.  The 6-point style originated from single seater race cars, where the sub-belts wrap down around the driver's legs and are attached to the car at the same place as the lap belts.  In a single seater the driver tends to be in a very reclined position so this works well.  But the 6-point 3" harness also works well in the more upright sedan seating position.  It allows more variability in the sub-belt mounting system.