Read. Not only will reading more help get your imagination running, it will help build your vocabulary. Words are the foundation of any writer's toolbox.
Keep an idea notebook. Jot your thoughts down as soon as they strike. Write down interesting snippets of conversations. Be sure to write down anything that inspires you in that moment, or what may inspire you in the future. You never know when an idea will strike, so make sure you always have your notebook handy (and sleep with it nearby in case inspiration strikes in the middle of the night).
Write. Whether it be writing in a journal or completing a few quick writing exercises you find on line, any kind of writing will help keep your skills (and your mind) sharp.
Write what you know. The words will come easier if you're familiar with the subjects, or even the emotions, behind what you're writing.
Research. Even if you're familiar with what you're writing about, it helps to do extra research. Your research will help add an air of authority and an extra layer of realism to the story you're telling.
Plot an outline. This can seem like a daunting, or even boring, task, but in the end it's usually worth it. It helps organize your thoughts and will help you to create plot points, or help you to decide where to place important information within your story.
Rewrite. Your first draft won't be your best draft. Keep plugging away.
Proofread. Check sentence structure, grammar, spelling, and such. These finishing touches will help polish your writing and make it more professional. After you've done your own proofreading, have a second pair of eyes (a friend, neighbor, classmate) look over your writing. They might spot something you missed.