John Smith has WHOLE WORKOUTS dedicated to relaxation. One of these is roughly 5X150 with a cone placed 50m from the finish. These are run fast but the last 50 is TIMED and you are REQUIRED to run relaxed. This workout is done most weeks for the whole season.
Another one done is 3-4 X (80+60) with a walkback rest and a longer rest between sets. The emphasis is on form rather than actual split times. BTW, Tyson Gay (with Drummond) did a version of this before his 100m heats at the OT in Eugene.
The way you learn the technique of relaxation after 60m is to DO IT IN PRACTICE.