Find a person or group of people that you admire and attach yourself, do your workouts with them. For me it is the trainers at my gym as well as the individuals who push themselves. When you are surrounded by people who want to be the best they can that mentality rubs off and you want to be the best you can be too.
With or without verbal encouragement from such people they will inevitably help keep you motivated by proximity. You will want to strive to catch-up or keep up with them. The relationships built in such a way keep you coming back for more.