Writing a synopsis of your novel to entice agents or publishers is one of the tasks writers hate most. I know I'd much rather write an entire first draft of a book than write a five-page synopsis. Yuck! To help you along, I found this nifty entry in a blog that summarizes the process nicely.
My own thoughts: First, best to write it after the novel is done. Don't try it during your first draft, because things may change and you'll have to re-synopsize, which is evil.
Second, keep it shorter rather than longer. Short and snappy.
Third, reveal the ending. You're not trying to create suspense. The agent/publisher wants to know that you can plot things out successfully and satisfy your reader.
Fourth, I start just slogging through events A-Z. Then I go back and make it sing.
Fifth, touch on themes and characters, but mostly you're trying to tell what happens.
Sixth, reward a drink or some chocolate when you're done. You deserve it.