One of the most important things you can do is appear credible. Most big companies forget that they did their most interesting things when they were just a few people with an uncertain future. When a big company wanted to visit us in our early days, we got a bunch of our friends to come hang out in the office so we looked bigger than we were. Along the same line, an introduction is much better than a cold call. (What big co? I'll help if I can.) And find a way to convince them you'll be around for awhile (we had to have our investors call a ceo once).
Getting a meeting, by the way, is about 10% of the work. Big cos often have difficult political landscapes and multiple decision makers. You have to figure out who wants what and convince them all they will get it. Also, big cos often except a discount for 'taking a risk' by working with a startup.