There are a couple ways to do this, but I am sure that there are more.
One way is to have an insulation contractor blow in Cellulose into the walls. Sometimes the existing fiberglass in the older homes have enough give to pack down and allow new cellulose in.
Another way is if you plan on residing your home, you can install a rigid foam board, either White Foam or Foamular. The Foamular will net you a 1 point higher R-value over that of the white foam. Example; 1" White foam has an R-value of 4.0, where as the Foamular 1" has an R factor of 5. This is the route that Iwill take when I reside my house.