For flash in studio I use an external flash meter to be able to trigger and measure the light.

Outside, an incident meter is great and in the majority of situations you can use the reading as is, otherwise I set the in camera meter to spot, measure off a highlight area on the face, and use from -2/3 to +2/3 exposure comp depending on complexion and reflectance. Pasty-complexioned white people in early Spring get +2/3, African Americans, East Indians etc with dark complexions get -2/3 comp, and people with olive skin (no tan) get exposed as the meter reads them. Of course your horse sense will kick in after awhile and this becomes automatic.

I don't like or use iTTL, though there are many who get great results. I also don't use automatic flash - I learned the old way and I am most comfortable with manual flash.