First is to reach out to several of your celebrities. I'd suggest coming up with several different offers that are fine-tuned to meet specific hot points: a boost to their ego, their need for some privacy or thirst for attention. In exchange for different levels of endorsements, you could offer short-term rehearsal time for a reduced rate or charge the same rate covering a longer period of time (i.e., "We'll give you three months of preferential studio time for the cost of one month.")
In terms of the different kinds of endorsements--an easier sell may be just the use of their name (or mention of the movie they were in for privacy seekers) in your marketing materials. Next up would be use of a quote and their name, with the best level (with a higher giveback on your part) allowing use of their head shot, name and quote.
To juice things up even more, you could sell one or more celebs on doing a short 30- or 60-second video testimonial for you, which you can post on YouTube. Of course you should also ask for specific airing rights to post on your site and to use in e-mail blasts. While it will take a small investment of their time (and of course the video team's fee), you can use this to help brand your firm and sell your services in so many online and print spaces.
Also, since most actors are looking for new ways to get in front of more people, an online video testimonial serves their own purposes.
Another sales plus for video: You can hyperlink the video back to your site and then hyperlink their name to their own site or a site of their choice, another way to promote their talent or unique interests.
Of course the celeb endorsement is one tool to use in an overall strategic marketing plan and multiple campaigns. Suffice to say that a strategic use of social networking spaces, online ads, SEO-enriched tect on your website, strong coding on your site, networking, direct mail, special events and PR can help you supersize your revenue stream.
Best of luck!