Smoking ciggies and alcohol and drugs, I overdid to the point of illness when I was younger. By default, I quit, but I re-introduced them gently after something of a detox. Now, I try to do them in moderation. Sometimes I fail... It's normal, but generally I am good. I try to keep myself in check by remembering the misery I had to go through.
Refined sugars, I got off that years ago, because I realized the effect it had on my body. I started with experimentation-- leaving it out of my diet, eating it again, etc. I realized it made me feel hazy and acidic.
I completely stopped eating it, like a hardball stance, and this helped stop my body from being used to it. That was an important step for me. After that, I started eating the occasional cake, dessert, sauce, with refined sugar. My standing rule is that I will not eat corporate, mass-manufactured stuff. This basically eliminates all sickly sweet shit and leaves me with the quality fresh stuff. I love eating, so this is important for me. Generally, I lay off the sugary stuff, but when I do have them, I follow them with a cup of tea or hot water.
I stick to this by bringing raw sugar when I travel (for dipping those stick rice cakes in, mmm), leaving a bottle raw sugar at some of my favorite establishments (like my favorite hot cacao joint, they use it when I order).
I stay away from all artificial sweeteners.
I try not to deny myself anything I really like. I find that going for quality of quantity has generally allowed me to be healthier.