As DavidK indicates, consistant session work is very hard to come by. Things like jingles, etc are run through marketing firms who either have in house contractors/composers or who hire outside contractor/composers. Those people in turn already have a network of musicians (1st call, 2nd call, etc). - or more often than not, use samples, etc. If musicians are used, it is very common that a musician is hired to contract the other musicians - in this case the contractor already has 1st call players, etc.
Artist recordings normally have one musician who acts as the contractor - (same thing happens with tours) and again, that person already has a network. Some smaller studios that focus more on singer/songwriter projects may use session players (although samples and loops have taken away most of that work) - but you still need an "in".
So....how does a musician become 1st call for contractors??? Network!!!! You need connections who respect your playing and are willing to hire, or recommend you. In almost every case, session musicians paid a lot of dues (when work was much more available) - playing in numerous bands, subbing in bands, playing small sessions for little on no money, etc.
Like DavidK - I did a lot of session work in the late 70's and early 80's (as a session drummer) - only to lose most of it to samples and sequencers (in the later 80's and 90's). However, every session I ever did came from connections I'd made through hundreds and hundreds of gigs - and in most cases, it was my ability to be dependable, friendly and profession more than my chops, that got me the gig.
Some recordings of you playing and some bio package (and certainly business cards) can have some value - however, nothing will replace playing with as many people as possble in as many musical genres as possible - you never know who may offer you the next gig, but your chance increase with every new musician you work with.
. Just to state that studio work like that is dying down alot!