I have not had my RNY surgery yet, but have had much done some extensive research about this because I feared same. In the general info session I asked this exact question. My doc was doing the presentation that evening and in his opinion if people follow the (his) rules, they will not gain the weight back.
* No added sugar or alcohol
* Only 3 meals/day and those are not to be shakes. He feels continuing to drink shakes is like drinking water too soon before and after a meal....the nutrients do not get a chance to get absorbed and will "go right thru you". This leads to more hunger and then the snacking begins.
* Diet full of nutrient rich foods. He said many people will make good changes when they are working with the nutritionist but then those fall by the wayside after surgery and the less desirable foods creep back in.
* No snacks- if you feel hungry enough to need a snack, your meals are probably not nutrient rich enough-revisit your choices.
* Keep up with your water & protein every day
*Ceasing exercise will diminish success

His comments do not pertain to the other available procedures as our discussion was around the RNY and I don't have enough familiarity with those to know if these points apply to those or not.

I don't know how far out from surgery you guys are, but I'm 4 years out and let me tell ya it aint easy. I thought I would never have a regain, but here I am 37 pounds heavier than my lowest weight. I never reached goal, but I had/have a lot of excess skin around my belly and my arms. I did notice at around 2 years out I could hold more food and it's almost like I "fogot" how to diet since I didn't really have to think about losing weight for so long ~ it just fell off. Literally. I wish you guys the best! I hope you don't end up like me!! Feel free to add me as a friend. I need all the support I can get!