After the novel is done, you must decide whether or not it needs a prologue. If it would help clarify or carry on the plot of the story, then it is a necessary evil. When you write a prologue for a novel, you must keep some things in mind in order to make it work for you instead of against you.
Write a prologue for a novel as though it were its own short story, with a beginning middle and end. The only difference would be that you cannot resolve any conflict in the prologue.
Have a strong hook at the beginning of the prologue, but don't believe that this means you don't have to have a good hook at the beginning of the book. Intrigue the reader with a strong entry into the prologue, and then an equally intriguing first chapter.
Change the feel of the prologue and make it stand out from the rest of the novel. Write the prologue from a different point of view or set it in a different time frame from the novel itself.
Add some background information for your character or story line in the prologue. This should be done only if it is absolutely necessary and it should be written with a flare that matches the content of the novel.
Read many different prologues to get ideas for your own. As every writer knows, in order to write well, you must read many other writers and genres to hone your craft. Treat writing a prologue for a novel in the same manner.
Use a bit of the book that you cut before and couldn't reintroduce. Sometimes you come up with a real gem of a phrase, but you can't find a place for it in the novel itself. Try tweaking the phrase and expanding on it and then write the prologue around it.