Really know yourself before entering ANY relationship (including friendships!), know your absolute deal breakers and TEST them. You may think you won't accept XYZ and next think you know your in XYZ knee deep. Also forgive yourself if you do!
However, my one lesson I got loud and clear is that if I'm not into someone, get out early! I stayed in my last relationship because he was a faithful man. My biggest fear in a relationship is to be cheated on (possibly because of my abandonement and trust issues). As a result I stayed out of fear of other men. Of course now I'm paranoid up to wazzoo but still better than lack of love, I'd even go as far to say...nah, too mean. Let's just say, I was NOT into him.
ETA- Disconnect from the exes for at least a year IF you are going to be friends later on. Otherwise disconnect completely! Tough lesson to learn for me.