Jingle work is a very tough business to break into. Unless you are on staff for a major agency (or contract with an agency) it is hard if not impossible to get a shot at the "national accounts". Naturally most of the national spots tend to be shopped in the major media markets (LA, NCY, Chicago)
I've never done any national spots, but have done local stuff for quite a while. In most major cities there tends to be a few key people who get the majority of calls from agencies for large local accounts (the McCartney's in that city). Often, the people who provide the music are not only 1st call from the agencies, but are also 1st call session players and normally work with (or sub with) various bands in that market area.
The jingle business is very in bred - when an agency has a good writer they tend to not only send most of the work to that person, but they are very protective of that person (so the competition can't get at him/her). Once you can get "an in" with an agency (and of course assuming you can produce)you can get consistant work - at least until that agency becomes "old" and losses it's edge (and it's accounts) which almost always happens after a few years.
