My high score is 5127. Like many of you it drives me crazy that the dictionary is so narrow. It is so weird that you can use proper nouns but polygamist isn't in their dictionary.
For my x's I always try the following: exhibit, exhale, exile, exit, oxygen
For zs: froze, frozen, gaze, gazing, blaze, blazing, etc
I add -ing or -ly to any word I can for extra points. I wish there was a place to play it online, I feel like I'd do so much better at the bonus rounds with a real mouse or touchpad.