Boy can I sympathize! Especially when I'm having "one of those days". A technique isn't working, you get frustrated, and tense, and it gets worse.
One way I've found to relax during practice is to smile, laugh, and talk to my uke as I'm doing the technique. Talking helps me shift my attention from the contact point, and instead of doing "steps 1, 2, and 3", it somehow blends together and ends up working the way it should.
Another thing I've had to have pounded in me is to BREATHE! Nice, deep, relaxed breaths timed to the rhythm of the "dance". It's hard to breathe normally when you're tense.
Hope this makes sense and is a bit helpful.