I had master Enchanting and Alchemy before I trained speech, so I just made a full set of equipment with the maximum barter enchantment and repeatedly bought and sold expensive items with a merchant. The more expensive the item, the more speech experience you gain by trading with it. You will lose a lot of money with this method, but if you're far in the game chances are you're already loaded anyways. In the end, it didn't make a dent in my finances since I just used the merchant, now overloaded with gold, to sell my extra and useless expensive items that I would have been forced to take a huge loss on otherwise.