Excerpt from LIVESTRONG.com

Overview
Getting anyone to do anything — particularly if neither you nor they are sure they want to do it at all — is an exercise in patience and, all too often, in futility. When you want to get someone to exercise with you, be prepared to hear an outright no, negativity, doubt or laughter. To motivate someone to exercise with you, you must pull out all the stops. Instead of emphasizing how it would help you, make it worth her while — emotionally, mentally and physically. Appeal to her desire to get fit and have fun in the process. Consult your doctor before beginning any new exercise regimen.
Offer a Compromise
It can’t be your way or the highway. If you prefer brisk walking to cycling or yoga to Pilates, there’s a good chance your friend will have her own likes and dislikes. Be forthcoming and explain that you would like to participate in some of your favorite activities — but are willing to engage in hers, too. Make a plan to switch each time you get together and make sure that when it’s your turn to do her favorite activity, you don’t whine about it. She’ll pick up on your negativity and that could be the beginning of the end of your exercise alliance. Embrace the opportunity to learn something new — and best of all, remember that you get to do it with a friend.
Support and Encouragement
Everyone has low days. Acknowledge that during your motivational speech to get your friend to exercise with you and emphasize the beauty of having moral support. You can even concoct a list of words or phrases, mutually agreed upon, that you will use on each other as motivators. When one of you is feeling down, even to the point of not wanting to exercise at all, activate the phrases you know will get each other moving and into a better place.
Plan a Trip
Make it a commitment to each other that you’ll celebrate your biggest milestone by taking a trip together or doing something you would normally never consider doing alone, like going on a retreat or to a spa. Something like this will never work unless you create a timeline. It can’t just be an off-the-cuff statement like, “Maybe we’ll do something together after we’ve lost weight.” Make weight-loss goals that are not so ambitious that one or both of you will quit from exhaustion or frustration. Write down reasonable dates by which you expect to reach certain goal weights. And once you’ve reached the halfway point, book your trip and make the down payment. A big expenditure is a great motivator.
Do It for Charity
Most everyone has a soft spot for the needy or sick. Ask a friend if he would be willing to be your exercise buddy for a good cause. Pick a favorite charity or group and sign up for a 5- or 10-mile walkathon that’s several months down the road. Plan to get together with your exercise partner a couple of times a week to train. Pick a name for your team and buy matching T-shirts imprinted with the name of your team and charity. Altruism and solidarity are motivational blockbusters.
References
- FitDay: Why You Should Have a Workout Buddy; Maria Faires, R.D.
- “The Interval Training Workout”; Joseph T. Nitti, et al.; 2001
- “Mayo Clinic: Fitness for Everybody”; Diane Dahm, M.D., and Jay Smith, M.D.; 2005