All this!
Attending the national AMSSM meeting. They hold a fellowship fair during the meeting so you can meet with almost all of the programs in the country.
I would also add that you need to be good at your current specialty! I'm going to assume you're going into FM since you're posting in this forum, but I think the most important part that's often overlooked by some is that you have to be a good family doc. If you're struggling as a resident then why would we take you as a fellow. Do well in residency, get to know your local sports medicine physicians (hopefully you have one on faculty), and try to do as much event coverage as possible (HS sporting events, local half/full marathons, rodeo, etc)