Flexible hours can work really well; most offices are blissfully quiet at 4am. Also the lights can be off!
Reserving a conference room for a few hours is great. You don't even have to be alone; a small group who are all working on the same thing at the same time can be immensely productive this way. Here's a good example: http://rachelbythebay.com/w/2011/08/24/projectdarkness/.
Basically, find all the ways you can exert some control over your environment and use them.
We don't have enough conference rooms for actual meetings, so that's rarely an option.
Really, I've only managed to mitigate the problem a bit, not solve it. Would love to hear anyone else's tactics. I'm seriously considering going freelance again mostly just because it means never having to work in an open plan office.
I'm hypersensitive and easily distracted so cutting off sound is a must. I also combined it with the pomodoro method for additional boost of focus. I'm now thinking of making it an online app to help others.
Even though I won't be able to fully hear their voices, I get distracted. Sometimes I doubt that I am very vulnerable.