This game is all about wasting the other team's time.
I start most matches by blazing a trail to the front line. I do this because nine out of 10 times, my teammates do the opposite and start painting our territory. If I can meet the enemies at the middle of the map (or ideally, while they're still at their end), and if I can spray at least a bit of paint and maybe throw a desperation grenade, it'll waste juuuuuust enough of their time that either 1) my teammates will have caught up and continue to distract them, and/or 2) my teammates will have painted in a good portion of the map behind me.
This is more or less my priority on every map except Saltspray Rig and Blackbelly Skatepark, where the objective instead becomes taking the high ground and muddying the ground near the enemy's side of the tower.
Arowana Mall is also interesting. You have the middle halfpipe with the two scaffoldings on either side. If you can take one of those scaffoldings and avoid snipers from the opposite and adjacent platforms, you can keep the halfpipe in your team's colors and repeatedly plant bombs in the blind spot on the other side of the enemy's ramp.
None of this is failproof, of course. I've still lost many matches doing this. But these seem to be good priorities during the first half of a match.
When it comes to the second half... I sometimes have success trying to push as far into enemy territory as possible, but I'm starting to think it's better to backup your teammates in the middle and push as a front toward the enemy.