There are three things that for me are key for great sex
1) Love the person you are having sex with - clearly love and sex can be seperate things but when you have them both it's so much better
2) Like the person you are haveing sex with - this one is a often a lot harder espessially if you have been in the relationship for a long time
3) Trust the person you are having sex with - there are more to sex than just intercourse, and sometimes an understanding partner who you are willing to please even if you can't "rise to the occasion" often when you stop thinking about your pleasure and focuse on her pleasure at least in my experience most of the times I have been ablet to "rise to the occasion"