Here's a great way to amp up your workouts: Find a training partner! Not only will you both be more inclined to go to the gym on a regular basis (or workout outside the gym by running, biking, swimming, etc.), but you can workout much safer as well. A training partner can also give assistance by "spotting" you during strength exercises and providing feedback on your form.
You can also compare notes on what you've accomplished and set goals for each other -- including rewards for sticking with your routines. If nothing else, it's more fun with a friend!