Personally what I do is, slightly bend the knees. I keep the back locked, I can't really go 90 degrees. I lock my elbows at my sides with lats tensed, that's my grip.
I focus on pulling with the elbows, as Oldfella said, no wrists, nor biceps, the more you lock the arms in place the better IMO. The most important thing to me is to breathe, squeeze at the top, which to me is between the belly button and chest. I use a reverse grip.
Then down and exhale. I use a lot of mind: muscle and squeeze as I do them.
It should almost feel as if you're expanding and flexing the lats as you work them. Back btw, is the one bodypart that everyone compliments me on.
As far as close grip, only for pulldowns, and I lean back as I do them. But I hardly ever do those, it's just weighted pullups and bent over rows for me.