Maintain a collection of lyrical ideas and constantly add to it. This will come in handy when you have a melody and you can't think of anything that fits, and it will keep you from falling back on lyrical cliches.
Sing your melody out loud, using gibberish or whatever words come out. This will help you to determine which vowel or consonant sounds work better throughout the melodic phrase. Just because a word or two fits the number of syllables you need to fill in your melody doesn't mean it will necessarily flow out nicely when sung. It should also make sense within the context of the feel of the melody and the rest of the lyrics in the song. If your melody sounds morose, sing words out that echo the sadness of the lyric, not something happy or silly, unless you're trying to convey some sense of irony. Sing whatever comes naturally. Sometimes a part of the melody will lend itself to particular words. Go with them if they sound right and fit the melody perfectly, even if the words clash a little bit with the rest of the lyrics or don't even make sense. Many people listen to music for the vocal melody and skip over the meaning of the lyrics. In Vanessa Williams' "Save The Best for Last," she sings "Sometimes the sun goes 'round the moon"--a false statement, but it sounds nice and fits the melody. Your lyrics don't even need to be real words, take, for example, Hanson's "MMMBop" or The Police's "De Do Do Do, De Da Da Da." Use whatever works.
Make compromises. You can take pre-existing lyrics and fit them in with your melody, but, in most cases, they won't match up perfectly. The melody should usually take precedence over the lyrics, as the tune of the song is more important in pop music. Be willing to cut a word or two here or there to make it fit. If you have to, change the melody a bit. Use this to your advantage at different parts of the song. For example, add a few extra words in the melody of the second verse of a song to change the melodic shape and surprise the listener.