Bow in with a smile and no expectations and accept what happens.
I like this advice. A while ago I read this book
and one of the key things that stuck with me was Cheng's piece of advice to his students to "invest in loss" when practicing with partners. In the case of the book it was push hands but you can apply it to anything.
When I'm having a tough time in practice, not being able to hold my center or make anything work, I try to stop trying to make it work
and just do as little as possible. Let the technique spring up spontaneously from the energy I'm getting from my partner and not care whether or not it actually succeeds or makes them do fancy ukemi. Keeps me humble and keeps me from getting frustrated.