You might find some useful advice by searching on this forum:
http://www.abrsm.org/forum/index.php?s= ... howforum=2
http://www.abrsm.org/forum/index.php?act=SF&s=&f=7
http://www.abrsm.org/forum/index.php?act=SF&s=&f=28
where, IIRC, I've read some discussions about effective ways to practice.
It seems that a lot of people recommend practicing hands (and pedals) separately and slowly and then putting them together and speeding up.
While you're waiting for some expert advice, here's something to get the ball rolling ... though perhaps you shouldn't take much notice of me because I'm not a good enough player - and perhaps that's partly because I've never felt comfortable with that slow/separate thing ...
But, to describe what's usually worked best for me, I'd use the analogy of the way you see pictures build up on a website - at first, big squares with limited detail, and then more detail added in layers. That's because I progress better if I start with a rough overall idea of how it actually feels to play the piece, and at the right speed - even if that means playing dozens of wrong notes in the initial practice sessions. Then, as far as possible, I try to learn pieces at something close to the right speed - that's because I find that my hands tend to address the keyboard rather differently at different speeds, so playing fast isn't the same as playing slowly but faster. I also prefer not to break the piece down into separate hands, etc, unless I have to. Of course, the odd passages that I can't do at a reasonable speed do have to be taken slowly, perhaps just to make the notes sink in or to see how the hands interact, etc - and perhaps be played with hands separately, etc. I think I'd find ALWAYS practicing hands separately to be counter-productive to the effective learning of a piece, as well as a serious waste of time - I prefer to reserve that approach for only when I can't manage otherwise.
Unfortunately I'm impeded by being an poor sight reader, and that can interfere with what I just described, because there'll be pieces I can play without much difficulty, but which are too hard for me to sight read. So (while a piece is still unfamiliar) I sometimes have to play some sections slowly, even if I'd prefer not to, because I can't sight-read them fast.
One area where care might be needed is being sure not to get used to the sound of different speeds in different sections - eg if you can play some of the piece at the right speed, but are playing other parts slowly for a while - you mustn't learn unconsciously to expect the easier parts to be faster. In any case, it's worth checking occasionally with a metrome to make sure you're not playing one section faster than another unless you meant to.