I agree with David. Practice to the point you are physically exhausted. Don't forget to breathe though.
Actually, I think David was mostly talking about pre
aikido. In that case, I'd have to agree with Mr. Hacker:
[snipped...] The key to learning how to appropriately release muscles doesn't lie in physically exhausting the muscles themselves, but in learning to control the job orders issued to those muscles.
Doing hundreds of repetitions to the point of physical exhaustion is more of a test of spirit and will than a path to correct technique. It can be a very useful tool, but when muscles are fatigued it's no longer really a choice to relax them.
It also goes without saying that I don't have time to physically exhaust myself in the fraction of an instant before an attack occurs. It's kinda like needing to stretch out before I can throw a decent kick. :-)