Sometimes it depends on what paint you're using.
Tamiya acrylics benefit from a little thinning to help it settle, but not as much thinner as you would use when airbrushing. As Andy said above, just a little clean thinner to dip your brush tip in occasionally when painting. It also helps the "flow" if you dampen the brush slightly in the thinner before you dip it in the paint. (just enough to moisten the brush - not wet. Dip the brush tip in thinner and touch it to a tissue/paper towel to wick it off before dipping in paint.)
Likewise, Vallejo Model Color paints are, to me, too thick to brush paint straight out of the bottle. When painting small areas, I will dispense a drop of paint and then pick up some distilled water with the brush and thin the drop of paint to the desired consistency, then paint in the same manner as above, periodically moistening the brush with the water before dipping in paint.