There are several ways I use, depending on the tracks.. For tracks with return rollers, I apply CA to the return rollers from 10 o'clock to 2 o'clock. Then I insert dowels between the tracks and the fenders. This pushes the track down around the rollers...
For tracks without them, I use black sewing thread looped around the axle of the roadwheel and the link to pull the center link down, almost touching the appropriate roadwheel in the case of tanks like the T-62 (3rd Wheel), or down to the first roadwheel for Tigers and such...
Just be sure the sag you're trying to make is appropriate for the tank/AFV you're doing... Some guys put way too much in, as if the idler has been deflated completely, and if it were a real AFV, it'd throw a track at the first turn...
Stay turned here.. I've got a Marder on the bench that I'm about to add sag to... I'll post some photos in the next day or two...