With at least Mandarin, you can do even better and include the tone marks over the vowels.
On a Mac (recent OS), you just need to hold the vowel down and you'll get a list with diacritics. Unfortunately, that doesn't work for the umlaut/trema in combination with a tone mark, though.
For shortcuts, use the US Extended keyboard, activated as System preferences > language & text > input sources. On that tab, also make sure "keyboard & character viewer" and "show input menu in menu bar" are checked. Then you can go to the flag and select "show keyboard viewer" for assistance on getting the keys. For the trema (umlaut), use "v", as in alt + e followed by v.
On Windows, one way to get the umlaut/trema is to activate the US-International keyboard input. See
http://symbolcodes.tlt.psu.edu/accents/codeint.html for Windows 7/Vista. With the international keyboard activated, type the quotation mark (shift apostrophe) followed by "u".
Unfortunately, there doesn't seem to be an easy way to get all the tone marks with a keyboard, though. Carlinga is freeware that requires no installation (you start it up when you want to use it) that provides easy-to-use key combinations for typing all kinds of characters. See
http://catology.boisset.eu/Carlinga.
HTH
Ben Barrett
Seattle, WA