Keep Sex Unconditional
It's too easy to carry resentments, worries, and daily life struggles into the sack. There are movies made about revenge sex and tit-for-tat sex for a reason. My sex life improved when I gave up any and all expectations beyond enjoying how it feels in my body and connecting with another person on an intimate level (and also having a lot of fun). Keep sex unconditional. It gets too messy if you don't.