I. The basics.
A. OEM suspensions: standard, sport/ZSP, M-sport/performance. Stiffer springs, dampers, sways as you move up.
(1) Change shocks/dampers. E.g., Koni yellows/FSDs, Bilstein HDs, BMW Performance (see sticky). Ride will feel more controlled and stable. Price for parts about $400-700; labor $350-500; moderately difficult 4-6 hour DIY. Sticking with OEM springs usually fine, but changing springs an option to consider (see #2 below).
(2) Change springs. E.g., H&R, Eibach. Ride will feel stiffer, less roll, more bounce off bumps. Usually lowers ride height/ground clearance. Price for parts about $250; labor about same as dampers -- change of springs should always be accompanied by change to a matching damper.
(3) Change springs and dampers as a coilover set. E.g., KWs, Bilsteins. Most have some adjustments for height, and some for dampening. Allows for more flexibility, change settings for different conditions. Parts $600-2500. Labor same as above, $350-500.
(4) Other components: Strut and chassis braces, Camber adjusters, Alignment, Corner balancing, LSD ... Other nice stuff to firm up chassis and provide fine tuning of how the rubber meets the road.
C. What mods are best for you? What parts should you buy? What labor and adjustments do you need?
Here is where I am hoping that members will provide helpful advice and comparisons. Post away!