Sometimes very creatively. First, try the easy path and call the office and ask the receptionist to pass you through. They're VERY skilled at telling you to go away nicely so be polite and get your pitch down to one line to answer this question:
"What is this regarding?"
"I'd like to talk to them about XYZ"
You'll probably be told to take a hike, or "He's busy, I'll pass along your message." and you'll never hear from them. But sometimes you get through so always start there.
When/if that doesn't work, guess at their e-mail address. This is actually my personal favourite which I've used successfully - Trade secrets! - and send them an e-mail that explains what you'd like to talk to them about and gives a time that you'll call (Offer them an opportunity to suggest a different time) and make sure you call at the time you specified. Eventually you'll get the address right and you probably won't get a reply but it's crucial that you call at the time you said you would.
Then all you have to do is call at that time. You'll reach the receptionist again and now you have a perfectly acceptable answer to the question "Is he expecting your call?". You can say: "Yes, I was in touch with him last week and said I'd follow up with him today at this time."
That's all they need to hear and they'll put you through to them. That'll get you in the door, the rest of the selling process still applies. Keep going, never give up, you'll win eventually. :)
Figure out what works for your personality and what you feel comfortable with and go with that. Everyone is different and some sales people I know call more frequently than I do (Or could with my style) and have found ways to not cross the annoying line.
Just keep at 'er. I like to think of sales as an art of the persistent. It can be a lot of fun and intensely frustrating all at the same time! A lot like programming... hmmm... maybe that's just me.