For the past few months I have been using this website/app that has literally gotten me to the gym 4x per week EVERY WEEK. It has been so useful for me that I wanted to make sure people knew about it.
It works like this: you make a commitment to exercise X days per week (I chose 4). Then you choose a dollar amount that you will be penalized for each workout that you don't make (I chose $10). Every time you go to the gym you "check in" on your phone. You have to stay for at least 30min for it to count (the app uses GPS to make sure you are where you say you are ... if you try to leave early the app will buzz.). At the end of the week (Sunday night) your credit card is charged for however many work-outs you missed from your pact x your penalty amount. The best part of all is that this money is then split among all the people who completed their pact. (The amount you get depends on how many days you commit to, so someone working out 7 days a week gets 7x more than someone working out 1 day a week.) The actual amount you make is measly (so far it's been about $0.50 per workout), but I find the incentive to not be charged $10 incredibly effective. You can find out more info on their website, but there are all sorts of exercise facilities in their database (spinning studios, etc) and you can always change or cancel your pact for the following week. I can't rave about it enough.
The website is GymPact dot com. (Please note that I have absolutely no affiliation with this site, I'm just a really happy user!)
All the best,