I'd say healthy communication is the key to a great, long-lasting RP. Honestly, it's the same for anything that involves two or more people, but it can be easy to forget or ignore. If something comes up, if there is a problem with something that happened in the RP, then just talk about it, bring it up in a non-confrontational way. I think a good portion of RPs die just because one or both parties fail to communicate properly or there is a breakdown in communication.
Be open to feedback, don't be afraid of saying no if something isn't to your taste. If you're afraid of bringing something up to a partner, then there is likely something wrong, so try to figure it out if you want to keep the RP going. Or if you're not comfortable with that, don't be afraid to end things. It's not worth tormenting yourself over it if you're just not feeling it anymore.