1. Constant online presence. It's not as good as actual "hallway time", but it's as close as you can get. Whether this is accomplished via AIM, Skype, IRC or whatever probably doesn't matter... whatever the team likes.

2. Face time. This is crucial. Now and then, the remote employee should spend a week in the office with the rest of the team. Ideally this would be once a month; once every two months is as far as it can probably be pushed. If a team member isn't worth flying around this often, they're probably not essential.