Generally speaking, the off-season is supposed to be used to recover from the long season. However, some athletes fall into the bad habit of overeating, focusing only on strength training and short-changing their cardio conditioning. By the time training camp comes around, they find themselves a step or two behind. (Ask the Experts! How Should I Eat in the Off-Season?)
Approach your off-season conditioning with a plan that combines strength and speed. This will minimize any needless weight gain and improve your strength and endurance. (See How to Get More Out of Your Off-Season Workouts.)
Being strong but slow won’t get you very high up on any sports team’s depth chart. Build strength and gain quickness, speed and agility by incorporating at least one conditioning session into your weekly routine—even if you lift weights four or more times a week.
I’m not talking about hours of monotonous hamster-style cardio on a treadmill or recumbent bike. You need to mimic the stops and starts that most sports require, so that means interval training. (See also Tabata Training: Quick Workouts for Amazing Results.)
Try the following drills:

Hill Sprints

Hill Sprints place your body in a safer position than flat ground sprints, because the angle of the terrain reduces impact on your knees, lowering the risk of pulling a hamstring.
After a thorough warm up, start with 10 minutes of sprints (30 seconds up, one minute walking down). If it’s cold out, make sure your legs are warm and ready before starting.
Slowly increase your number of sprints over time. If you start at 10 minutes, bump it up to 15 by week three and to 20 minutes by week five. At that point, you can take a week off and reset. This doesn’t have to be an endless catabolic marathon. You should top out at 20 to 30 minutes of sprints at your peak.
Watch NFL safety Steve Gregory perform Hill Sprints.

Sled Pushes/Pulls

This is a great conditioning exercise if you have a sled handy. Commonly known as the Prowler, this little contraption can push any athlete to his or her limits. It allows for adding weight, increasing resistance even more.
Learn Sled Pushes from NHL All-Star Mike Green.

Cart Pushes

A poor man’s version of the Prowler. Find a shopping cart in a parking lot, load it up with heavy objects and sprint forward while pushing the cart for 45 to 60 seconds or until your speed starts to decline. As with the previous two drills, start by doing a minimum of 10 minutes and progressively increase your time as the weeks progress.
Watch Craig Kimbrel perform his “Junkyard” Sled Drives.

Sideways Sprints

You aren’t always moving straight forward or backward in any sport. Quick bursts side-to-side are necessary at times to dodge an opponent or catch a ball.
Set up two pylons (or cones) about 30 yards apart. With body facing forward, shuffle between them, pausing to touch the top of each one as you reach it.

Bike Intervals

If you’re stuck indoors due to weather, sprints on a bike can be as effective as the four drills described above. Unlike a treadmill, you can go full out on a bike without risking an embarrassing fall. Also, the stationary bike forces you to maintain a decent pace between sprints, as there’s no cruising momentum like you get on a bike outdoors.