Use Your Gym Membership
If you signed up for a gym months (or years) ago and haven't been going, you are essentially throwing your money away. Take advantage of that membership by making going to the gym a habit.
Find a time that realistically works for you: if you find yourself constantly staying late at work or filled with other evening obligations, then going in the morning is probably your best bet (and these tips for waking up earlier can help here, too). If you just can't force yourself to wake up before dawn, then plan on bringing a change of clothes to the office and going right after work.
If it's a matter of motivation, try these 10 tricks to get motivated to work out, such as making a date with a friend, saving your favorite shows to watch on your iPad while hitting the cardio machines, rewarding yourself afterward (like with your favorite protein smoothie or a few minutes in the steam room), or throwing on your favorite fitness gear.
And here are some ways to be proactive: set mini goals, create a fun playlist, and track your progress so you stick with it.