One quick tip, never, ever throw anything you've written away, no matter how bad you, or other people, might think it is, put it away in a drawer or a folder on your computer and leave it until you are struggling to write.
When you look at old writing with a fresh perspective, weeks, months or even years later then you can often do something with it, even if it's just taking a line from it or a turn of phrase and often it'll spark new thoughts and new directions in which to take the jokes.
I often look at some of my old jokes, which are generally terrible but now, with a more experienced eye, I can generally see what needs tweaking and use them as a starting point for writing new material.