There's a seemingly endless number of ways to decide which route to take. Do you start with an around-the-world fare? Base it on weather? Throw a bunch of darts at a map? Plan your route around a particular festival or your favorite hobby? Do you go East-to-West or West-to-East?
We started off with the darts-on-a-map approach, which didn't really narrow things down all that much because there are only a small handful of places on the planet we don't want to visit. From there we started looking at weather factors, which changed a few times as our game plan changed. Budget played a small part; we've always wanted to visit Italy but now is just not the time to go to Europe if you want to stretch your US dollars. Our route was also influenced by expat friends in far-flung places, as well as CouchSurfing contacts, as it's always nicer to stay in someone's guest bedroom and get shown around town by a local.
In the end, we decided to focus on a specific region (SE Asia/India) rather than trying to literally circle the globe. There's no way we're going to cram everything into one trip, so we're not even trying!
We'll be posting detailed info on our route through SE Asia & India in map and calendar format as our plans evolve.