Just to add to what Scott says, you can clean the brush in tap water (I usually have a bit of clean water in reach) and then dry it a bit on some paper towel or whatever and wick the tide mark off the surface before it dries and even use the damp brush to feather and clean the edges as you go. Just don't do the whole model thinking you can go back later (like with enamels and oils) because it dries so fast.

I do this with any enamels or oil wash, obviously you have way more time with those than you do with the acrylics but if you apply the wash and clean the edges as you go, it works fine. If the model is bone dry, the tide mark (which is nothing more then the edge of the puddle due to surface tension) will seemingly 'etch' itself into the surface quickly and be hard to rid. If the surface is pre-dampened with tap water (damp not not soaked) the tide mark will clean up much more easily.

Similar to the way you can pre-dampen a surface with mineral spirits and apply the oils without fear of staining the surface.