looking at this
http://www.powerathletesmag.com/archives/seven/pullups.htm

John Allstadt says the best way to focus on bringing the lats out (during pullups or chinups or pulldowns) and working it to it's potential is to:
use a thumbless grip.
imagine your amputated from the elbow up
your hands and forearms are hooks attached to the bar
and you pull through your elbow.

Another lat technique or rather a shoulderblade retraction is to imagine gripping a tennis ball between your shoulderblades.

I think thats a good way to focus on your lats, but i wonder whether theres other ways to visualise your other muscles into bringing it out. I hear people mentioning squeezing or flexing your chest muscles when benching to maximise it, but i dont know how exactly.

Hope it makes sense what i posted. :D

Mind muscle visualization is HUGELY important. Once you figure out the way a move is supposed to occur and get a feel for it- it's so important, for me anyway, to always have the visualization down pat.