Here's a great tip from Sheila Bender, prolific author of Creative Writing Demystified and the founder of Writing It Real, an online community for writers.
"Writing happens in short amounts of time--just a lot of them strung together! But when we feel blocked, a quick way to get unblocked is to remember how much you can write in ten minutes. Try an exercise like this one:
Describe what you see from where you sit using words that appeal to the five senses: what do you see, hear, taste (or remember the taste of), touch and smell. The senses are always bringing us huge amounts of information we often don't stop to describe and include in our writing. If you are stuck on what a character is thinking or doing, do this exercise from their point of view. If you don't know what you want to write about, do the exercise and then imagine someone you can no longer speak with because of loss or growing distances has enter your view. What do you have to say to this person from this place you both are right now? They to you? And you can always do this exercise allowing your character to be the person who drifts in to the scene. You'll be unstuck and writing in no time. The trick to unblocking, I think, is to return to sensory information--when you are describing what the senses are bringing in ,you make associations and this unfreezes the writer within."