Is there anything I can do to prevent bottoming out?

There are several things that contribute to bottoming out, one being implant size. The bigger (and therefore heavier) the implant, the more it will stress the tissues that hold it up. Saline implants may be more likely to bottom out because of fluid wave pressures. Textured implants might stick in place better but they are usually firmer. Wearing an underwire bra compulsively is also a good idea. 

There are multiple factors that contribute to bottoming out.  There are probably only two that your choice could influence: size and texturing.  The bigger they are the harder they fall is an old adage but true.  Unfortunately surgeon's have no capability to measure or predict how much weight your tissues can maintain.  Texturing of certain manufacturers may help minimize this complication but as you attested to there are trade offs in the "feel" of the augmented breast.  Why not have another experienced board certified plastic surgeon examine you and listen to both opinions.  Mom always said that no one ever died from too much advise.  Best Wishes

Drop out is when the implant descends well below the center of the nipple.  Patients with a weak breast fold, larger implants, and smooth implants are more at risk.  I recommend textured implants, reasonable size, and post operative bra support at all times for 6 months.  The most important, however, with a fold incision approach is to do a really good deep tissue repair with a long lasting suture like PDS.  I think this forms a kind of shelf for the implant to sit on until a capsule is formed.