I only say "on your left" to other riders that ride like they know what it means, otherwise you end up startling people thus creating more havoc than keeping quiet.
for anyone else I either
1. Move as far away from them as possible and pass them as quick as possible. Then I look over and give them "hello" as pass.
2. Ring my bell giving them enough space and time to compose themselves and pick a side. Then I do step
#1
this is worth every penny, when approaching "traffic" this bell is loud enough to give people enough to time to clear the path.
Spurcycle Black Bell. More potent and more streamlined. Just better.
Several riders usually jump into my draft and stay there just so that I can clear the way for them.