Just start at the top
My one bit of advice from watching others fail is to be sure you start at the top and disposition every single bit of stuff one thing at a time.
When I've done the massive collection and processing, I've found it difficult to get much done because everything I needed was in the one pile instead of covering every available surface in the room. So I had to plow through it in order to get anything else done.
I've seen people who don't have one big pile get distracted by all the stuff their still surrounded by and find the process so prolonged that they get discouraged and give up. So you could start on one side of the room and work around, but it takes discipline.
I think its best to collect ALL the stuff and go until you can't take it anymore. A couple of days of discipline will provide weeks, months or years of return.