If your house was built before 1930, chances are that the exterior walls are empty. If so, your walls can be drilled (usually from the outside) and filled with dense-pack cellulose insulation to R-14 (R-3.7 per inch). Dense-pack cellulose greatly reduces drafts in walls, especially compared to fiberglass batts, and provides excellent sound-proofing and fire-retardancy.
Post-war houses usually have a minimal amount of insulation in the walls (usually R-8 mineral wool batts). Therefore, these are not the best candidates to achieve a uniform distribution of cellulose with the dense-packing method. Furthermore, it is usually not considered practical or cost-effective to open up such walls unless the plaster or siding needs to be replaced. Other options can be considered on a case-by-case basis.