This has a series of activities. Start with the introduction, then try the tongue twisters and dictation exercises. If you have the Shockwave plugin for Netscape or Internet Explorer 3 or 4, at least 16MB of RAM, and a direct Internet connection, it's worth the wait. This lets you put vowel sounds together to end up with a dipthong. It requires Shockwave, so it takes a lot of memory and processing power.
Some help is free, but more detailed advice is available for a fee.
Check the Accent Tutor Online for free activities. To get more, you need to subscribe. The animation is cute. The information gives explanations with a Korean context. You'll need to print out the page, cut it into 'dominoes', and use it with a friend or two to learn syllable stress rules.