Contrary to popular wisdom, drying in high heat does not shrink clothing, in fact it will restretch some of the fibers shrunk by washing.
Wash it in as hot a water as possible, using as much agitation as possible. Hang it dry or use a gentle/no heat dryer cycle.
If that doesn't work, you'll probably have to get it tailored. Denim will only shrink so much and even then it will tend to stretch back out a bit in time.
Or you could try eating a lot of pasta and ice cream.