It is very easy to run through a perfectly good work out, get to the end, and talk yourself out of finishing those last couple of reps. This is where most people make their mistake. You have to stress your muscles in order to change them. Make sure you keep your form throughout the entire exercise. Yes, that last set is a killer, but your body will thank you for it. When the sets begin to get easier, challenge yourself by adding more weight, an extra set, or a modified version of your exercise to increase performance.