I second the kissthisguy.com recommendation. What I usually do is go to Goggle with a phrase I KNOW is in the song enclosed in parenthesis (this makes Google treat that phrase as a single word, and tends to give better results, I've found that once you give Google more than 3 words in a search the results get pretty vague) followed by the word lyrics. Example:

"do you promise not to tell" lyrics

...which gives me lots of sites that have the Beatles song I was looking for.