It is acrylics that are water soluable. You can get extender solution for acrylics. Painting in a somewhat humid environment can help a little. Normaly when I paint with acrylics I moisten the paper, canvas or wood I'm working on first. Not so helpful for minis.
Personally, I prefer oils. I have a fairly good medium worked out consisting of linseed oil, varnish, and turpentine. A japanese drier can be added if your impatient. At one point I made my own paints for a class. Its really easy, just add your pigment to linseed oil and grind on a slightly sand-blasted glass plate using a glass paint grinder. Scrape the paint from the glass paint and place in a tin tube for storage. This method has the advantage of allowing you to control a lot of the paints properties, something store bought paints don't allow you to do so well.
Finding the right cleaning solution also helps, usually mineral spirit or turpentine. But it is still important to shampoo and condition the brushes after rinsing them out with paint thinner. Leaving the paint thinner on the brushes ruins the hairs.