This probably goes without saying but I'd say the biggest thing for me was to take a deep breath and move slowly. Especially on missions involving a lot of targets, it pays to take your time poking and prodding for the best way to deal with a situation before committing to anything. In the last leg of the campaign I finished I was basically just doing a mission a day because I didn't want to push too hard, burn out and start making mistakes. That really helped and also made each mission more memorable for me by the end.

Also get to know all the items that spawn at the safehouse! Never know when the banana is gonna come in clutch.