For the smoothest performance, it helps to wet your paint brush before the first dip into the paint. This helps the bristles glide smoothly, reducing bristle marks or uneven deposit of paint.  If using any type of water-based paint, wet the bristles with water. If using an oil-based product, dampen the bristles with mineral spirits. In either case, then gently blot the bristles on a clean rag to leave them damp, not dripping. Now go ahead and paint as usual.