Don't join a gym this January. Seriously. Gyms aren't so much about helping you get fitter as they are about raking in profits by maximizing their total membership, and minimizing their active base.
And I say this as a hard-core gym-goer who's spent many thousands of dollars/euros/pounds on personal training and gym memberships over the years.
Take a yoga class. Try a self-defense course. Start running. Take the stairs. Get off one stop early. Buy some weights or a suspension system. Attempt the 100 push-up challenge. Try bodyweight exercises or P90X.
Any one of those options will set you back less than the cost of a one-month gym membership. And your sunk costs will be considerably lower if you flake out by February.
But whatever you decide, you might want to wait before you tell anyone lest the Gollowitzer principle undermine your very best intentions. There's a very watchable TED talk by Derek Siverson the subject, if you're so inclined.
Writing goals down, though. That's cool. If somewhat unscientific. Writing your goals down in a Moleskine probably wouldn't help you get them done any faster, though. Or if pen-on-paper isn't your thing, or you prefer recording achievements (ahem, Gollwitzer) iDoneThis might be of interest. And if even email is too old school, Streaks.
May you achieve at least 50% of your resolutions, etc.
This post first appeared, with links and other content not included here, in the Galavant Times on Jan 5 2012.