For leather I use Dodo Juice Leather cleaner, it is the most effective one I used (and I've spent far to much on this kind of stuff
) you will need to use a conditioner, I use Gtechniq L1 Leather guard, but in my experience there isn't too much difference in conditioners.
For fabric cleaning I use Chemical Guys Lightning Fast, it is the most effective one I used for removing stains, but it also caused a dramatic better effect on the red carpets of my S2000 which looked fine before. I haven't got any experience with it on seatbelts and headlinings though, I never clean a headlining unless there are really obvious stains since getting it wet can promote sagging.