Use high quality stain and brushes. Inexpensive brushes may not hold enough stain for an even application, and low-quality stains dry too fast for you to avoid streaking.
Dip the brush in to get sufficient amounts of stain, and do it consistently for each stroke. Too little and it will be drying while you go back for more, and too much will cause you to brush endlessly while trying to cover a large area. Practice will teach you the right amount for the particular stain and the brush.
Stroke long, quickly, evenly and with the grain. Don't jab at the piece and don't start-stop-start when brushing.
Feather the edges, or brush them out to the side of the main strokes so that you won't be starting the next side brush strokes on bare wood.
Stop at natural points such as edges or where a piece of wood joins another piece.