Honesty
Breaking up is really hard to do. I think what's really important in a relationship is honesty. Often times, relationships simply don't work out because one, if not both, of the parties are DISHONEST. This is why, at least according to some doctors, is the reason why our society today just isn't the same.
If you're in a hetero-sexual relationship---be honest. Don't mess around or screw up. It's hard, yes; but it will make you happy in the long run. If you see yourself slipping, catch yourself. Get counseling, talk with your significant other, or just avoid the situation. Don't make life harder than it already is.
If you're in a homo-sexual relationship---the same applies. Be honest. Set expectations and boundaries for yourself. Also, talk with your mate and verify the grounds for your relationship. It's not worth losing that perfect someone just because you messed up once. Keep the communication lines open. Honesty is the key.
Obviously what I am saying is that in ANY TYPE of relationship, you really just need to be honest. Actually, in many loving relationships, the other partner will be MORE LOVING and respectful if you have the guts to tell the truth. Hope these words sit well with you.