One of the best ways to get fit for summer, is to increase the intensity of your fitness routine. Running on a treadmill for a half hour straight at the same pace just isn't going to cut it. Sure, you may get some results, but you won't get that ideal physique you want for by the time summer comes around.
One of the best ways to increase exercise intensity is to group a variety of exercises together into a circuit. For example, instead of doing 3 sets of just lunges, add in rows and a cable woodchop exercise in that same set. And if you're feeling tough, add in a short burst of high intensity cardio, like sprints. Do this for your other exercises and you'll notice your workouts will be shorter, but you'll be surprised how tough your workouts will be and how many calories you will burn.
Keep in mind, that any form of interval training or circuit training is much more intense than a traditional workout. Always seek proper medical attention if you are unsure you are able to participate.