Practice, practice, practice... But when you practice, don't just go all out. Two things that are most important to work on. 1st work on technique, make sure you are moving your arms and legs in the most efficient manner possible for the stroke. You may need a coach to watch you and critique your technique, but correct technique will get you the most speed. 2nd, your muscles rely on oxygen to operate. They get tired when they don't get enough, so work on breath control and stamina. Make sure your breathing technique is correct for each stroke and then work up stamina by swimming as long as you can under water and then trying to improve that distance each day. Good luck.

get some ropes and put them in the pool while u practice backstroke and try to go straight and fast by putting ur arms straight and circle them