If I'm addicted to heroin, should I try to visualize all the horrible things that will happen to me if I don't overcome my addiction?

That would be a negative motivation.

Now, I'm going to speculate a little, so I want to clearly mark the following as speculation:

I have had one person who told me they had significantly cut back their dose of a painkiller they were addicted to, by removing the negative feelings that prompted their use of it. This is just one example, though, and there are way too many confounding factors. (Inlcuding the part where they had only reported cutting down, not quitting entirely yet.)

All that being said, I would guess that the distinction between being addicted to something and simply enjoying it a lot, is the same as the distinction between negative and positive motivation. People might start smoking, drinking, or drugging for the purpose of receiving pleasure, but once they're an addict, the overwhelming motivation is to avoid withdrawal and/or the lousy life they might otherwise have to face.

I expect this is equally true for addictions like excessive internet usage, for example.

Anyway, I'm just putting the "speculation" disclaimer here because I don't treat people for drug, sex, alcohol, or gambling addictions. I've helped people quit smoking, sure, and I've cured my previously compulsive addiction to lesswrong, but that's about it. ;-)

I wouldn't recommend pain to people who have learned to self-medicate with addictive behaviors. I would recommend enough 'gain brain' motivation to encourage whatever positive action is required to combat the addiction. (eg. Finding someone to tie you down and inject you with suitable chemicals till the worst of the chemical addiction is gone.)

I don't think addiction IS a form of akrasia.