Quick tip: Athletes sometime get sucked into the rhythm that's bouncing around in their brain, which means the music you're listening to can affect your speed in the pool, for better or worse. Keep that in mind.
Obsess Over Technique
If you've ever had a coach or a seasoned swimmer look at your stroke, you may have received some advice on things to keep in mind going forward.
An ongoing lap swim is the perfect time to obsess over these details. You have nothing else to do.
Focus on that steadier kick. Or reaching to maximum length. Or making sure you're not shortening your stroke. Perfect practice makes perfect.
Mix Up Breathing
If you're extremely comfortable breathing to the right every other stroke, spend the next two laps breathing to the left. Or even better, try bilateral breathing for 100 yards—you may develop enough comfort to do it indefinitely (which is a good thing).
Being a versatile breather keeps your stroke balanced—and it allows you to mix up your swim.
The fewer strokes it takes you to get across the pool, the better. As Kevin Koskella wrote, "The world's best swimmers are faster than you because they travel further with each stroke, not because they are moving their arms faster."
Spend the time counting your strokes and working to chip away at that number. It will make you a more efficient swimmer, and do a great job of passing the time.
Find a Different Pool
Sometimes, a change of scenery can do the trick. If you've gotten used to the 25-yard pool in your neighborhood, seek out an Olympic-sized pool and do your next long workout there.
Mix In Different Strokes
Nothing will fight boredom better than mixing in the butterfly, backstroke or breaststroke to your workout.
Put a 200-yard IM into the middle of your workout. Or if you don't know all the strokes, learn the backstroke and substitute in a lap amidst all your freestyle laps. As Koskella writes, "learning proper backstroke and mixing it in to your workouts can help your freestyle hip rotation...if you're only going to learn one other stroke besides freestyle, backstroke should be it."
Watch Your Neighbors
Sometimes, you can learn a lot by watching the person in the lane next to you.
If your neighbor is consistently faster than you, watch a little bit while you swim and try to figure out why. What's he doing that you're not? Is his stroke longer? Where does he end in stroke? Is he kicking differently than you?
Watching your fellow swimmers out the corner of your eye is much more interesting than staring at the blue line at the bottom of the pool—plus you might learn something from your peers.
Enjoy the Daydreaming
For many of us, swimming laps is a break in an otherwise crazy day chasing around kids, putting in a full day of work and juggling all of life's responsibilities. So you're bored. So what? Let your mind drift off and appreciate the alone time. There's not enough of it in this world.