Lots of people moan about how badly treated they are by recruitment agencies, and I am not defending any of the poor practice that there is out there. However some of the criticism comes from a fundamental misunderstanding about what the agencies exist to do and how they operate.
Recruitment firms specialise in certain jobs/professions/sectors. You will be wasting your time if you approach people who don't fill roles in your niche.
You need to remember that you are not paying the recruitment agency, the employer pays them so they really struggle to spend time on you as it is not bringing in any money unless they can place you. So back to finding the recruiter who fills jobs in your target niche..and you may need to get down to finding the recruiter who fills jobs in the company you want to work for. So you have to do your homework and prep.. don't expect them to be circulating your details around all their clients .. this is an employers market.
Then you need a fabulous keyword rich CV and pitch which tells them why you will get them a a 'hire' and thus commission. Then you need to build the relationship so that they know you are valuable to them. Then don't annoy them by being a pest and contacting too frequently.
They are struggling too, so be supportive and helpful to them!