When using bodyweight this is a fantastic way to improve the exercise and make it harder. During the movement of the chosen exercise, try slowing your pace down, for example when doing a Push-Up, slow it down, count for 6 seconds on the way down, then go up for 6 seconds. You could hold it in the middle for 2 seconds as well. When looking to build muscle mass, this is a great idea, on the eccentric part of the movement, so when you go back to the starting position and lower your arm on a bicep curl, this is the part which results in your muscle growing! The slower you go, the more muscle fibres you tear, and therefore more need repairing, and when they repair they come back bigger, which is muscle growth! Make sure you get that protein in after your session!