It's obvious plum130 failed miserably on his answer, when Paasche is actually the correct answer!! :) :)
A natural or sable hair brush works best for me (same brushes used for oils). Synthetic brushes (nylon, for example) tend to react negatively with the solvents in the enamels, and degrade fairly quickly, but are excellent for acrylics. In terms of brushes, I have found that you pretty much get what you pay for. Spend the extra to get a good brush, take care of it, and it should last a long time.
Gip Winecoff