Because it's so counterintuitive, negative splitting is a difficult technique to master.
The first thing is to keep a close watch on a clock during training. Coach Emmett Hines emphasizes that until you're trained thoroughly in negative splitting, you can't trust your internal sense of time. Although it might seem inconvenient to consistently check a pace clock, Hines suggests several options. If swimming near a clock isn't possible, you should use a watch. Or, if you can't see the clock from where you are, you should get prescription goggles [source: Hines].
As you've probably guessed, for negative splitting, you should check the clock at the halfway point. Coach Hines tells his swimmers who typically use flip turns to do an open turn at the halfway point if they have to in order to glance at a clock [source: Hines].
After checking the time at the halfway point, you'll have to remember it during the way back, and, if possible, do some quick math in your head to calculate double that time. As soon as you finish the second half, check the clock again. If you end on more than double, it was a positive split, and you took longer on the second half. Obviously, the goal for negative splitting is to finish under double.
But don't be discouraged if it takes time to beat your first-half time. Though you may put in more effort on the second half, swimming harder doesn't necessarily mean swimming faster. Many swimmers have a difficult time with negative splitting because they start too fast or don't actually increase their stroke tempo on the second half [source: Hines].
Once you have greater control of negative split training, you'll gain a better internal sense of your time and pacing. Ultimately, mastering pacing is the holy grail of this training.