Just build a pretty steep little kicker in into the biggest powstash you can find, make sure to throw straight back, tuck those knees, keep looking straight up/back till you see the landing. The biggest thing with backflips though is absolute commitment, if you hesitate at the last minute your gonna either land flat on your back/head or throw harder back with one shoulder and end up all crooked and out of control. Once you get over the commitment thing(definately the hardest part for me) they feel really natural and smooth.