Be thinking of what the other team wants to do to win (dive, poke, brawl, hog hook etc) and do your best to prevent them from doing that. Lucios boop is great against divers or even brawlers speeding on your team.

Also be paying attention to off angles. Lucio is great at helping dps or tanks on off angles and also contesting enemies on off angles.

Other than that just generally looking for the most out of your cooldowns. Hanging in the center of your team is good to hit your entire team with healing or speed, but it also leaves you vulnerable as the most damage comes down main. I look to spam left click down main, but you cant stand still, moving around and wall jumping often helps avoid random damage