Done a bit of this in the past. What sort of temperature are you getting the ali sheet up to ?? It doesn't need to be too hot as all you're trying to do is relieve the stresses in the material. One tip I can give you.... rub some normal hand soap (bar type, not liquid) along the sheet of ali where you plan to start the first bend. Heat the run with the torch until the soap turns dark brown/black, that's the right temperature to get the sheet ready for working. As you work the ali it will build up stress again.....you need to keep annealling the sheet as before or it will crack.
HTH.
