How to follow up and push myself on to top of the candidates poo

Midcareer, has interviewed for a job with a medium size company. I feel my qualification and experience match the job very well and I have gone through an onsite interview about one and half month ago, meeting the hiring manager and team for a whole afternoon. After interview, I feel that I am the person they are looking for, confident that I can make instant impact. But I haven't heard back from them since. How should I follow up with them and reiterate my strength ? By the way, I was hooked up with company through a staffing firm, and the recruiter worked with me left the staffing firm during the process.
Any helps are highly appreciated.