I assume you are referring to sag.
Workable indy tracks are easy, they sag from their own weight although plastic ones might need a little assistance (push 'em down with a pen etc.). The glue type ones are a little harder, you need to let the glue set a little (Time varys depending on glue.) so the tracks stay together but are still flexible and then create sag with a little pressure. Too much pressure will pull the links apart.
Rubber band tracks can be held on return rollers/road wheels with CA (super glue etc.) and if the they need to sag between return rollers you can use pins. Mark the hull at the lowest point of the sag and either drill a tiny hole to insert the pin or heat the pin and push it through (if you heat it let it cool before allowing the tracks to touch it). Cut off the head, paint it flat black and it should be nearly invisible.