Packing a silencer is easy, and should become a part of your regular maintenance. After removing the old packing, clean the core thoroughly, and make sure all the little holes are clear. Wrap the packing (fiberglass works fine, and is much cheaper than the stuff you buy at the dealer) snugly around the core, and use enough to make it a nice snug fit when sliding it into the can. I also like to seal both end caps with a small amount of silicone before sliding them completely into the can.
You definitely need to repack on a regular basis. When the packing becomes oil-logged or starts to blow out, not only does the bike get louder, but performance suffers as well.