I use to do more jingle (advertising work) but as outlaw suggests, most of the ad agencies (now referred to as marketing firms) have in house capabilities (just like you and I can now have a recording
studio with a laptop and a spare bedroom - so can the marketing people). So, a lot of that work has dried up. Even most of the radio stations that feature radio ads have in house
studios, so a business does not even have to go to a marketing firm - just go to the radio station and be willing to pay for the air time and a nominal fee for the recorded "spot"
It is almost impossible to get your foot in the door for any national/international marketing - but there may be opportunities on a local/regional level.
On a local level, I have selected a few companies that did not appear to do audio marketing (radio ads, etc.) and I put together "spec spots" (a recorded jingle for free, hoping to entice their interest). I arranged to meet whoever was the "decision maker" brought a "boom box" and played my "demo". In most cases, I walked away empty handed - in a few cases, I did get some business (then my spec work became paid work) and I could then also try to market "music on hold" services that I can also provide.
A couple of things to be aware of. In most cases, business owners and/or marketing firms want to deal with "professionals" - this means, they expect meetings to be held during normal business hours (if you have a day gig or go to school and can only do business after 5 PM - you've already eliminated most of your potential clients.
Also, when it comes to advertising spots, it is common to provide several different lengths (a 15 sec. spot, a 30 sec. spot, a 60 sec. spot) so you have to be able to edit your demo several ways. Candidly, I've written some really clever 60 sec. spots that were never used, because the advertising budget would only allow for 15 sec spots - so the clever verse+
chorus composition I slaved over came down to the hook of the hame of the company.
Which leads to anoth point - you need a hook (in most cases) something that highlights the name of the company/product) - think about all the commericals you've heard and disect what is the "hook"
Lastly - it is a very, very competitive business with realy talented people competing for the same projects (In my local market, some of the areas top musicians supplement gigging by doing jingles - and these guys are the best of the best in this market) - so make sure you are capable of producing at a very high level!