In my experience using a plastic bag immediately kicks in survival instinct and I'm still not sure how to overcome that instinct. So I could see that causing issue unless you passed out quickly. Adding in alcohol usually can make effects worse but at the same time could make you get sick, so I'd use that at your own risk. If you're not taking what you have right away then eating less to prepare could help potentially since the drugs might be more potent that way. And you could try to tie your hands so you can't back out but that's easier said than done and survival instinct can easily kick in there too. I guess you could prepare too by just knowing your triggers and knowing what to do so you don't get sick quickly or anything else that might have hindered you before. In the end I don't know anything foolproof and everything has its downsides. After all, I'm still here. I wish you luck and hope you do whatever is best for you, think it through plenty whatever route you take.