1) It sometimes helps to let the clutch out in neutral before engaging reverse. Never force the shifter. If it does go in smoothly, put it back in neutral and let the clutch out for a moment.
2) Do you know how to properly downshift? If not, you may have worn sychronizers. If so, rebuild or replace the gearbox. If you properly downshift, you don't need synchros.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)