Get a room-mate. Sharing an apartment with other people will give you at least a bare minimum of socializing and something to exchange a quick chat once or twice a day. Plus it saves money.
Keep social routine - meet with other people for diner, sports, chat, movie. Keep track when was the last time you meet with other people. Keep in mind the people enjoy being asked-out. Be casual, don't be pushy and let them decline without guilt, and they will be happy that there is someone that shows interest in hanging out with them, even if they are declining most of the time. Most people gets lonely, yet lacks courage and motivation to actively seek companionship. Going for a short walk or doing groceries together can be nice too. And you can be open about the fact that you're seeking companionship more activey, as you're working remotely.
Socialize on other events: conferences, user groups, etc. This is also professionally important. If you work in the office, you will get to know people that might like you and get you a job in the future. If you're working remotely you're risking being left out.
Radio in the background with some interesting talks can feel like hanging around people.
Gym is always good. Especially if you're working remotely. It gives you some schedule, motivation and happy feeling.