How do I prevent my new sunburn from blistering?

Nothing. If you haven't blistered in a short time after developing the sunburn you wont. The only thing you can do is stay out of the sun or use good sunblock to prevent the burn.
The die is cast. If you have sunburned your skin severely enough to cause blistering, it will blister; the damage has been done. People will tell you that their favorite folk remedy prevented it, but in fact their burn was not severe enough to blister in the first place. All you can do is ride it out. Then buy a good sunscreen (spf 50), and don't try to get a tan all in one day.
Heal w aloe . I agree with dr. Eastern. Aloe can help w healing. Aloe vera (aloe barbadensis) can speed burn healing by stimulating growth of new skin. Use of aloe vera can help to prevent keratinisation of new skin growth and thus it may help to minimize scarring & speed the healing process of burns and skin wounds. It stimulates the growth of new healthy skin cells.