In my first year of studying Chinese I learned (by doing it wrong in front of Chinese friends) that there are rules for where to put tone markings in words with more than one vowel. The official rules, according to Mark Swofford's very helpful site, are:

• A and e trump all other vowels and always take the tone mark. There are no Mandarin syllables in Hanyu Pinyin that contain both a and e.
• In the combination ou, o takes the mark.
• In all other cases, the final vowel takes the mark.

But I noticed the craziest little pattern. If you want to remember only one rule and one exception think of it like this:

• Alphabetical order dictates which letter gets the tone mark. (if it's "biāo" then the "a" is alphabetically first, so give it the tone mark)
• The only exception I've found is the "iu" combination. In this case the "u" gets the mark even though it's alphabetically after "i"
• Oh wait, there is another rather rare exception and that's "io" combination ("o" get's the mark")

So here's the way I remember it:

• "duì 对" in Chinese means "correct," so the alphabetical rule is correct for that word
• "diū 丢" in Chinese means "to lose," so the alphabetical rule has lost it's integrity (and face) for that word
• Since "io" is also an exception (xiōngdì 兄弟), "I owe" everyone an apology for saying there was only one exception

Still, since the "io" combo is very rare, and the "ui" and "iu" combos are easy to watch for, I've found the alphabetical rule to be the easiest to remember and use.