Using a Mac? Easy umlauts (ä,ü,ö) and eszett (ß)!
If you're on a Mac computer, you can download this keyboard layout to get easy umlauts and double S: http://hci.rwth-aachen.de/USGermanKeyboard
It will be the standard U.S. keyboard layout but allow you to easily type the umlauts using the option key. So you can keep it on all the time and not have to switch keyboard layout.
Once you've followed the instructions to install the new keyboard layout, to get ü or ö or ä, just hold the option key then the corresponding letter (just like you hold shift to get a capital, and a shift+option+letter will give you Ä, Ö and Ü). To get ß I hold option then hit 's'.
Just in case it helps, there is no need to install anything on a MAC nowadays:
Ä/ä, Ö/ö, Ü/ü (press "option" and "u" then the vowel)
ß (press "option" and "s")
Wombatua - This is why I suggested using the keyboard layout above. You don't need to switch between keyboards using the flag, it's always on, and you don't need to learn a new keyboard layout (Which is harder when you don't actually have the layout actually under your fingers)