If you are doing mawashi geri followed by ushiro mawashi geri, you could "cheat" and put your front (kicking) leg slightly over your centre line after delivering the mawashi geri, while turning your back foot. That way, your spinning kick has a shorter distance to travel. Not at all okay in a kata, and any sempai will spank me for advising it at all, but try it for kumite.
Asides from that, rotate with your entire body, from toe to hip to torso to neck - use your elbows to "pull yourself" around. Make sure to look over your shoulder, otherwise you will not get the impact zone right. Retract your leg after the kick, so you land in a balanced kumite no kamae. With ushiro geri, I find it easier to tuck the leg in as close to my standing leg as possible, so the knee points to the floor.
I hope that helps, and that my explanation is understood.
This was very general, of course. If there is anything specific, other than the roundhouse - spinning kick combination, don't be afraid to ask.