If you're going to use multiple grounds, start ground cables close to the local load (e.g. the R-C, or at least R, at each end, as many suggested to add a load, and I agree), and twist each signal-ground couple.
As monpjc said, differential signalling is the best option (it is also used, for example, for incremental encoders for motors, which work on long cables into a very noisy environment), and it adds the same number of cables as multiple grounds, together with some cheap drivers.