I found the best thing was to spend about a year just observing how my captains dealt with different situations. As you get closer to upgrade, ask try some of the captains duties such as getting the daily briefing, preparing the weight and balance (we're still old school) or doing a mx write up. Ask questions as they arise, and study your companies AOM/GOM and other manuals. Being in the left seat really isn't rocket surgery, it just requires more of a top down approach to things than just driving the airplane.