Whenever you use a drug, you gain addiction. The primary way of removing that addiction is to fly to Switzerland and buy a number of rehabs at the drug clinic there. This starts off pretty cheap, but the more times you visit, the more expensive it gets. However, there are a few tricks to slow down that increase.
The first thing to realise is that the effectiveness of rehab is directly related to the number of times you have rehabbed. The more $250k rehabs you have done, the less addiction they remove. However, it isn't directly related to the number of drugs you have taken; it is possible to have taken a lot of drugs and still keep your rehab bill (relatively) low.
There are several ways to slow down the inevitable rehab cost creep; none of these are game-changers, but over time they will have an effect. I talk primarily about xanax, because that tends to be the drug that is most used, but it applies to any addiction.
Join a faction with good specials
If you are in a faction with a high addiction special, your addiction will go up more slowly. At the highest level, Addiction-XXV, this will reduce your addiction levels by 50%, which is a great starting point.
Join the right company
The 5* special at a cruise liner or a nightclub allows you to spend job points to reduce your addiction level. A xanax is 35 addiction points, so in a 10* company, that is over a quarter of a xanax a day for 'free'. If you are in an Addict-XXV faction, that moves up to over half a xanax a day.
Make the most of your natural addiction decay
Every night around midnight Torn time, your addiction level is reduced. If you are going to rehab, try to make it early enough that you can take your drugs before midnight, so that the overnight rehab decay gets used after your rehab.
Don't rehab fully after a xanax overdose
You won't be able to take another drug for 24 hours; If you want to go rehab for the happy, you may as well just buy one, or else hold on until just before the cooldown runs out, as rehabbing fully will just mean that you waste that evening's natural decay.
Don't fully rehab
Just don't do them all. If you need 8 rehabs costing $2.00m to fully rehab, instead just use 7 at $1.75m and leave the last one. That rehab would probably not be fully used, so hang on to it until the next time. On average, this will save you $125k every time you visit rehab, and more importantly it will reduce the number of rehabs used by a half-rehab for every visit. That all adds up, as long as you start early enough.
Save up your addiction
If you are going away for a week or two, and you won't be able to log in - don't rehab before you go! Let natural addiction work for you over those days and come back to a much smaller rehab bill.