What is the best way to reach marketing guys of brands for advertising purposes?

Have replied on Quora but generally speaking my advice is that it's really hard to get through to these guys. They're busy, they get hustled all day every day.

However if you approach their media or advertising agency you may be able to get in with a recommendation.

What specifically are you trying to pitch in? Feel free to drop me a line to discuss approaches that have worked for us.

Direct emails?through linkedin? through lead generation firms?through digital marketing firms of brands?For an ex,assume a company like foursquare or an ad network like admob how do they reach advertisers(in this case only brands like walmart,pepsi,Levis,Subway)?What is the best way?