I run a website that generates sales leads for agents working in
financial sectors like banking, insurance, and real estate.  Our
customers include representatives from dozens of large companies.  We
would like to display the logos of some of these companies on our
website to show consumers some of the products offered through our
service.

What is the most appropriate and effective way to approach these
companies to ask for permission to display their logos?  We have seen
a number of our competitors using them so it seems that they are
willing to allow the usage under certain conditions, but we don't even
know where to begin in getting in touch with right people.  We want to
make certain that we operate in such a fashion that these companies
don't think we are trying to take advantage but rather that we want to
promote their products as effectively as possible.