Simple - you stop using it for a while (at least a month). I'm a recovering heroin addict, but for one whole year, I got into methamphetamine. I was already getting dextroamphetamine prescribed for ADHD and I was getting the 5 mg IR tablets (150 of them a month). I abused the shit out of those and when I ran out (it usually took about 10 days) I would go out and purchase crystal meth and continue on until I saw my doctor and got my script the next month. Anyways, to make a long story short - that year I abused amphetamines turned out to be the worst year of all my years in addiction (I'm turning 27 next month), but I started doing drugs when I was 14. Got into opiates when I was 16, heroin at age 19 began shooting it at 21. At 24 (2008) was when I got into the amphetamine shit.
It was the most perverted, insidious, and darkest year of my entire years of drug use (from 1998-present).
I stopped using it. Now if I take 3 Dexedrine's I get fucked up, whereas before I needed about 15-20 of the 5 mg tablets. I still get them prescribed of course, but I use them like I'm supposed to. Three years later and my tolerance to amphetamines has gone back to baseline.