Thin the paint about one part thinner to three parts paint (do this on a palette, don't thin the whole bottle)When brush painting, thin with Tamiya X-20A thinner. Do not use water, do not use Windex
Moisten your brush with clean thinner before dipping it in the paint. Damp but not wet - if it's too wet, wick off the excess on a tissue or paper towel before dipping it in the paint. While painting, repeat this step every third or fourth time you load the brush with paint.
Tamiya acrylics skin very quickly, but take considerably longer to dry and cure, so don't be tempted to paint over parts that appear to be dry. Doing so invites "roll-up", pulling up semi-dried paint in big ugly lumps. Paint one coat and then leave about 24 hours or so before recoating.