On the whole just let it grow naturally as they don't particularly like being pruned. Sometimes it may be neccessary to thin out the crown a bit but wait until it has lost all it's leaves before you prune. I didn't realise how much Acers don't like being pruned and trimmed mine back at the wrong time of year and thought i'd killed it ..for more than a year I just had a bare trunk and branches but this year it has put on lots of new growth .....phew!

If you mean a Japanese acer then it will not take kindly to being pruned right now. I did a bit of tidying up on ours last autumn and even removed a couple of large branches on one but only after it was dormant.