For watercolor, it's very important that you obtain the best brushes you can afford and use them ONLY for watercolor and gouache. If you use a good sable brush for acrylic painting or oils, you're going to be crippling some of the water-carrying and flexibility aspects of the brush. If you buy good brushes and care for them well, a good brush will last you a lifetime. If a painter is skilled enough to paint or draw well in any medium, they are skilled enough to do so in any other medium. Because in my mind, working in watercolor and oils is like wearing cotton or leather.
Acrylics are more like wearing polyester.
Probably the most important thing I could say is to just touch the brush to a paper towel before painting with it. This removes the bead of excess water at the tip, allowing for better brush control. Besides that, I would tell a beginner to try and use the least amount of water that will do the job, and to use the proper brush size, for the proper task. If you use too small or too large a brush, you're going to screw it up.