Any tips on negotiating with manufacturers?

Has anyone here negotiated anything with manufacturers and wouldn't mind sharing? I'm thinking real stuff, not digital.

Basically, I have this deal with (Chinese) manufacturers that I want to close but I'm interested in:

how far have you been pushing things down
how did you account for the samples before the bulk order - were they included as a total, what are the common practices etc.
what things to watch out for
do your deals end up wildly different from the initial quotations, etc.

Alternatively if you know of any reddit community that might point me towards the answer I'd really appreciate it!