Although most kids grow out of it, it may take a while. I was a closet finger sucker for longer than I'll ever admit. :-) I did stop, though, and your daughter will, too. But you're not completely clear whether she just has a habit of putting her hands in her mouth or sucking.
There is a REALLY yucky fingernail polish with a very bitter taste that my parents used to help me stop sucking my fingers when I was little. It seeps into the nails, so the taste remains long after the polish is taken off. Although I'd try other things first if she's sucking fingers, it might be very effective if she's just putting them in there frequently.
Before you go this route, I'd look at why she's doing this. Maybe she needs a distraction or a gentle reminder. If you can find the triggers, you can help her find other ways to soothe herself.
Here's a great article: http://www.mothernature.com/Library/Bookshelf/Books/50/10...
In case you need a last resort, I did a quick search and found the following that might be helpful:
http://www.stopbitingnails.com/
http://www.amazon.com/Orly-No-Bite/dp/B0001AQCWY
http://www.folica.com/MAVALA_Stop_for_r442_1.html (good testimonials can be found here)
Hope you're able to find something that works for you!
L.