Driver’s slouch is the worst.  Catching yourself and adjusting to a good position is half of the battle. If you are doing highway driving, use cruise control so you can move your feet around.

Elbow hang will definitely pull you off to the side.  Break that habit.  I tend to move my hands around a lot.  If only one hand is on the wheel for a while, rest the other one on your leg or somewhere neutral.  I find hands on the bottom half of the wheel allows me to keep my shoulders back and in slight external rotation to prevent slump.
If you can’t prevent slouching in your seat you might want to consider an extra lumbar support to help you stay in a good position.Make sure you are upright enough that you can easily see everything you need to see with your neck in a good position. 
Readjust frequently.  Fidget.  All of the mobs on sitting in an office chair apply here.  If it’s a long drive, stop and take breaks so your body can reset.  An ergonomic specialist once told me that the spine can only hold a static position for 20 min before the muscles start to let go.  You have to move/stretch/do something to reactivate those muscles on a regular basis if you want to stay upright.
Hope that helps a little.