Loads of excellent advice has already been given, so my two penn'orth...
Despite all the good advice urging you to relax, of course nothing makes it more difficult to relax than being told to - this will come with time so just try your best not to tense up on the controls when you notice bits of you are starting to ache!
Less is more - if you are learning in an R22 then this may seem impossible at this stage, but the less you stir the cyclic, the more economical you are with movement, the more stable and controllable your hover will become. Watch your instructor and the minute
pressures that he/she applies to the controls and try to emulate this (while on the subject, what do you experienced folk think about the whole "following through on the controls thing? I often found it more confusing as I was holding them so lightly so as not to interfere I hadn't a clue what was going on...)
Finally,
enjoy it! You won't know what you are doing any differently when you finally manage a stable hover, but it's got to be one of the best feelings in the World!
Oh, and thanks to Pat Malone, who visits these forums. He was alongside me the day I cracked it, so it would be interesting to hear his advice...
Keep trying
DBChopper