With yourself that is! Make bets with yourself with rewards. 

“I bet I can go from 10 sit-ups a day to 50 sit-ups a day by March, and if I do, I’ll get that top I’ve wanted.”

“I bet I can walk an extra block each day this week, and if I do, I’ll treat myself to a day of pampering.”

Whatever your reward, give yourself a challenging goal to accomplish. Keep at it, and it will soon turn into a habit for which you no longer need rewards.