You can use an acrylic thinner instead of water. That way the pigments are spread across more of the binding agent that hold the paint together instead of breaking up in water. If you're having trouble, you might need to get higher quality paints. I use a "student" grade artist acrylic (mostly because I'm an illustrator and it's what I had on hand). Good brands to look at are Basics, Amsterdam, or Daler-Rowney System 3. They're not as expensive as professional grade paints, but they're a step up from craft paint, and should give you better results.
The relative humidity where you are can also affect how the paint dries. If it's still tacky after a few days, though, I'd look into trying a different brand of paints.
enguinaspin: