A great way to add resistance to certain exercises is through the use of resistance bands. They are great to use when you seek an alternate to using weights or when you do not have access to a gym. A few different exercises which can be done to target certain muscle groups are demonstrated below to give you a basic idea.
There are a few levels of bands, ranging from easy to advanced. Easy being more elastic, thus not adding too much resistance as opposed to the more advanced band.
triceps overhead extensions
triceps push downs
Upright Shoulder Row
*Squats can also be done using the same position or by placing the band and your hands behind your head.