One of the two options: First option, make it hubris rather than addiction, similar to what Faith experienced in season 3. That is, the power goes to her head and she feels she's above all rules and limitations, that her life and power is beyond the understanding of most, and if she can bring people back from the dead and do all sorts of good things then she DESERVES to get what she wants, or at least the scales will always be in her favor (that is, "I do so much good that I can be allowed a little indulgence" that just gets worse over time).
The other option is to create a mystical drug (as opposed to magic itself being the drug). In this case, Willow taps out and she learns of this magical meth can can give her the power pretty much as shown. Unfortunately, just like with many drugs, it causes the body to become reliant on it. That is, the more she uses it, the more she needs it and the less she gets out of it so that she has to up the dose to get the same effect, and she's scared of losing her powers altogether. Naturally, a rush should come from this to add to the addictive qualities. For the most part they could otherwise play it as they did, just iron out the "very special episodes" stuff like Rack (even the literal loan shark struck me as less silly than how he was portrayed--though props to the actor, my criticism is only for the character), hitting rock bottom so fast (which begs the question how Rack stays in business, especially if there are many other dealers like him) and the stuff that comes off like Partnership for a Drug Free America crap that I rightly felt insulted our intelligence back when I was a kid.
I'm not saying recreational drugs are harmless (though of course there are many people even using hard drugs who keep their lives together that we don't know about because it's illegal for them to come forward, and I seriously doubt sketching junkies on the streets are contributing the majority of billions of dollars to drug dealers in the US alone every year), or that people don't hit rock bottom as Willow did. It's just it doesn't happen like THAT, and I found it extremely annoying in addition to feeling extremely forced by the writers.
I'd have found an intelligent look at addiction, especially to a mystical drug, fascinating, however. It just needs to come off as something different than written by soccer moms and Sunday School teachers trying to scare the kids that even doing drugs once will destroy their lives by using ridiculous stereotypes and situations like that. It would probably be best to show Willow dabbling in some sort of "magical caffeine" in the very first episode (and also Tara deeply concerned, but--for the moment--keeping the secret despite her misgivings about it) with Amy later taking Willow to get the much stronger stuff, and it would need to a a bit more subtle than with Willow taking "Dawnie" through back alleys (though maybe much closer to the end of the season it could've worked), but everyone could be concerned before it gets that far.