The best thing to do in my opinion is have someone do a thermal scan of the walls and find out exactly where insulation is needed first.
Once you know that, the easiest and most cost effective way to insultate them is have someone drill small holes at the top of the wall between the studs and blow in cellulose or fiber insulation to fill the caveties.
For more information:
Read "I have a soyfoam insulation estimate for the first floor walls of my 40 x 80 sq ft home built in 1976. Is $4250 worth it?" a Q&A answered by Cynthia Phakos.