practice, practice practice...........
Im using wargaming paint that is almost identical to Andrea or Vallejo paints, and the secret there is to thin it with water, but then ALMOST dry your brush on a towel. the ideal is to have the STARTof the paintstroke ALMOST dry as you FINISH your paintstroke. Sounds dumb, but belive me, with enough practice you will get it to work for you. A flow extender really helps these acrylic paints, Im using a small bottle of Liqutex Slo Dri from the paint aisle at Hobby Lobby, and that small bottle will last me for years at the rate Im using it.
Check out the tutorials here, I go here all the time and it has helped tremendously:
http://www.reapermini.com/TheCraft
Although the colors and subject may not be familiar, paint concepts are very similar.
You can build up your paint in thin almost transparent layers. Most new painters put on WAY too much paint and ruin the job. Here, less is usually more.
Games workshop also has some great brush tutorials that I have used, in basically unlearning all of my bad old habits.
HTH
Build to please yourself, and don't worry about what others think!
TI 4019 Jolly Roger Squadron, 501st Legion