Hiring a guitar instructor can help add a sense of accountability that could keep you motivated to practice. It's also nice that they can teach you good practicing habits and whatnot. But the social accountability effects are really not to be ignored.

It also helps if a good friend of yours tries taking up the instrument at the same time as you. I was lucky enough for this to be the case when I was first starting, and I found that there was a mutually-reinforcing effect that ultimately benefited both of us. Maybe you can convince a friend of yours to pick it up along with you.

After a certain point I think the habit becomes self sustaining in the sense that "practicing" no longer feels like practicing, it just feels like, well, playing. And then if anything the hard part is keeping yourself from playing when you really should be doing other things. Once you reach that point, I think you're pretty much set for life.