Resume and Contracting help

I have been unsuccessful at finding permanent employment but have worked for three different contracting companies for the last 18 months. Most of them have been short contracts from a couple days to a few weeks or a month. The longest contract I have had was 6 months.

How should I reflect this on my resume and when applying for jobs? I have worked for a bunch of different companies with these contracting firms and was wondering if I should mention the companies or just the contract agencies.

Also, how should I address dates? Should I list just the dates that I worked or should I list the date for the whole time that I was accepting work from the contract agency whether I was on an assignment or not?

Thanks for your advice.