Infinite XP
You'll need to have a speech skill level of 50 or higher before attempting this trick. Approach Old Ben who sits by a fire near the King headquarters in Freeside. Persuade him to escort you to the local bar. For beating the Speech Challenge you'll get 61 XP. Follow him back to the bar and let him sit down. Speak to him again and you can do the Speech Challenge infinitely for unlimited XP.