Exercise – This seems to help many people, myself included. Any form of exercise or physical exertion can get you out of your rut and help your creativity begin to flow again.
Think About Titles – It doesn’t really matter what kind of work the title is for, just that it speaks to you as a writer and sparks your imagination. Type a random word into the search engine of your favorite book or movie store website, browse shelves at your local library, or check out your own DVD selection. Make sure you write down any ideas the titles generate (brilliant or boring), you never know what might be tweaked to become your next masterpiece.
Prepare Ideas in Advance – Keeping an idea book is a great idea. You just scribble down ideas or random thoughts as they come to you. You might even consider keeping a couple in different locations (your car, work, home, etc). Not only does it give you something to write about when you sit down, but reading through your old ideas can be a great way to help with brainstorming new ideas.
Try a Different POV – If your main characters are normally young men, try writing from the point of view of an old woman, or a dog, or a flower that sits on the windowsill day after day. See if you can freshen up old ideas by changing the focus of the story.
Use Everyday Objects – The general idea of this is that writing fodder can be found in everything. What if that toaster came to life? What if the person who made your carpet realized that his customers were dying off one by one? Why not write an article about how pillows were invented or a manual for aliens on how to use the toilet?
Reread What You Have Written – This is especially helpful if you are picking up from what you have written the day before. Going back over what you have already written can help get you back into the story and characters. You can also catch mistakes you have made when you reread with fresh eyes. Beware of falling into the trap of spending all your time editing when you should be writing.
People Watch – Watch people in a public place with a journal close by. Write down things that catch your attention or conversations you overhear, and of course any ideas that may strike you. Try not to look to creepy or stalker-ish when you do this.