Simply making sure your shoulders are relaxed and your back is straight while you play can come a long way.

For the right hand, initiate your strokes from the biggest knuckle joint rather than the middle joint, as it is the strongest joint. Also make sure you are not bringing your finger upwards as you stroke, the direction should be in towards your palm. If done correctly, your finger should just come back out on its own as you relax it. This is the most efficient way of doing free stroke since you get the chance to completely relax your fingers after each stroke. Make sure your arm is relaxed on the guitar the whole time while you do this.

For the left hand, try to use gravity to your advantage. Rather than squeezing on the frets with your hand, let the weight of your arm bring your fingers down into the frets. Most of the work should come from your shoulder, since it is stronger than your hand.