What happens when you don't use the knowledge you have? It deteriorates. Why do we let our knowledge deteriorate? Because we lack reason to use the skills we have been given. If you find this happening to you in the programming space, perhaps you should program something for the fun of it.
A friend of mine wanted to brush up her skills in C#, but couldn't think of anything to write. After hearing her dilemma, I created a problem for her to solve.
To see more practice problems, see the label of the same name.