The thing is that I'm just always enamoured with so many MMOs that I can't just play one and nothing else. I have to be a "juggler:" a player who balances online worlds. With limited time at my disposal and a desire to be in three or four MMOs at any given time, I've done a lot of experimenting with different ways to juggle titles.
There is no one best way, I've found. It depends on how "fair" you want to be to your MMOs and whether it's a priority to you to give each of them more or less the same attention and time. Each of the following strategies has pros and cons, and if you're trying to handle two or more titles, you're simply going to need to figure out what works best for you and your allotment of time.