Stretching out the clear plastic on shoes

Ok, I need some help. I bought some shoes for the competition and I need to have them broken in so I can put them on mid performance. I got a basic clear pair w/no ankle strap. They have the clear plastic band. I can wear them but I have to get into them with a bit of force, once they feel warmed up and I take them off. they got back to thier orginal size. Any suggestions on keeping the plastic stretched out?