"No thanks"
Believe it or not, the above works most of the time!
If I have to self-protect harder, I simply say "I'm not drinking right now." I don't go into complex explanations of my "alcoholism" (I'm not a fan of that word BTW; I don't consider myself having been physically dependent - but that's another topic). If someone really pushed, I just explain that I'm focusing on my health, and alcohol derails my plans.
You will be AMAZED at how few people are pushy about drinking, how many non-drinkers there are out there, how little you actually have to explain, etc.
Many athletes do not drink. All you have to say, really, is "I'm training for a triathlon."
If someone says "you were training for a triathlon before and you still drank" (I don't think anyone will actually say this, but just in case...), you could say "I'm trying to take my training up a level." Bam. End o' story.
Seriously, almost no one has asked me why I'm not drinking. Because of my profession, I am often at receptions, etc., where the alcohol is flowing. I love being sharp as others get sloppy. I drink juice or bubbly water or fancy combinations of juice and bubbly water.