Michael Davies, a part of the Linux.conf.au (LCA) conference talk review committee, spent a session at this year's conference talking about how they review talk submissions and choose which ones to accept for this large Australian open source conference. While he spoke specifically about LCA, his tips are largely applicable to those interested in submitting a talk proposal to any conference.
The LCA papers committee consists of experienced LCA attendees and volunteers who are both active in their communities and familiar with the conference. They try to keep a balance from year to year of fresh voices with experienced reviewers. The repeat reviewers help ensure that the conference maintains the same feel from year to year, so they use the previous year's papers committee with a few new members. They also consider the balance of the committee members.