You need a good GPS signal to get correct sensor readings that follow your body's' motion as closely as possible. Keep your phone in a relatively airtight area, where nothing obscures the signal. A small chest pocket or a pant leg pocket would be best. Snowboarders have told us that they always put stuff in side pockets (breast and pant). Skiers fall to the side, so they generally like to keep their phones in chest pockets.
Do not keep your phone in a backpack or in any other place which might obstruct the GPS signal or have space to move around, which will give false g-force readings and result in bad jump data.