Is there a way to prepare my skin for extra large implants?

At size 34 D each 100 cc’s of implant corresponds to 1 cup
size change. Increasing your implants by
an additional 400 cc’s will take you up to a H.
It is highly probably that these implants will be too large for your
body, your skin will progressively stretch, thin and your breasts will droop
requiring revision. This will not be a
long term stable product and you will require additional surgery after you have
irreversibly damaged your skin. Best Wishes,Gary Horndeski, M.D.

You have already stretched the skin and breast tissue significantly, and I'm surprised that you are not already close to an F with your current implants. Although I'm not usually a fan of implants that large, there are some ways to help minimize the risks of nerve damage, thinning skin, rippling, and early sag (ptosis) when large implants are used. If you want to go as big as you can, using a spectrum saline implant with a port that can be filled and deflated after surgery can allow you to see what different sizes look like and how much stress they put on your tissues. If you don't want a saline implant, you can always have them traded out for silicone later on. Some surgeons will even do this in the office under local anesthesia. Also, ask your surgeon about an allograft (tissue sling) to help strengthen and support your tissues so they are less likely to thin and sag over time. Good luck! 

but if you have a well developed capsule, consideration must be given to removing the anterior wall for full skin expansion.  But as mentioned, you have to accept the long term risks of big implants and you can expect procedures in the future to deal with thinning and drooping of your tissues as well as the need to downsize your implant.  If having the dream cup size will make a difference in your life, then go for it.  If not, consider something less.

Be careful what you ask for. There is little you can do preoperatively, however rest assured that if you go with 800+ implants with such a petite frame that your breast skin will surely stretch in the coming years... and it's unlikely that you will be happy.

The BRAVA breast enlargement system is used to enlarge breast skin envelope prior to fat transplantation for breast enlargement.However your plan to have 800 cc breast implants is too large and carries many risks. 800 cc breast implants have a much higher risk of complication including extrusion or loss of implant and capsular contraction.Please consult a board certified plastic surgeon who is very experienced a breast augmentation and reconsider your plan.