When you clean the injectors have them done on a cleaning / testing rig. Avoid add-to-the-petrol fixes - they don't. More frequent oil changes will reduce the carbon / gum deposits.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i