If you have the ability, using a torch while bending helps you get a tighter bend. But, you have to be very careful doing this.
Also, a tip when bending aluminum cold: The slower you make the bend, the harder the metal in the bending area gets, making the force required to do a tight bend increase by about 4x over the course of a 90 degree bend. To combat this, once you have scored and started your bend, you should make the rest of the bend in one smooth motion for best results. This may take some practice. Using a torch, as i mentioned above will combat this hardening effect, but you have to be careful not to warp of oxidize the aluminum too badly.