With the rate at which new software languages and technologies are appearing, its a tough job for a software engineer to keep up with the latest developments and trends as well as keeping existing skills sharp. Just as you feel you master one language or technology, a new one comes along and your apprenticeship starts over in this new field.
One of the best ways that I have found to both learn new languages/skills and to keep existing skills sharp is to practice each skill as much as possible. For programming languages this means regularly writing pieces of code in each language in the toolbox. I try to use each language that I know at least once a month for languages that I have become proficient in and at least once a week for languages that I am learning.
Dave Thomas describes this concept in greater detail in his CodeKata series.
I find the use of programming puzzles and challenges as excellent ways to find concise coding exercises that don't soak up to much time but yet provide opportunities to hone & develop skills as well as often providing opportunities to learn about new libraries, packages, algorithms etc.
Some of the best coding problem / challenge sites that I use are: