What really makes for a success in APB is good team dynamic. Having someone with a well equip long range like the Obir/Obeya/NHVR with plenty of STAR's (there quite potent right now) and at least one OCA/PMG/JG user for the close in scrims. That makes a good fluid team that can really thrive in most situations, Of course each end of the spectrum suffers when in open or close quarters combat but that brings me to my next point.
Use your supply kit to change weapons often! If you find yourself stuck dying over and over to a kid with an OCA hiding in a dark alley, switch to a Shotgun or an OCA and deal with him, have a specific weapon from Long/ Medium/ and Close range that you are relatively good with and practice with them often. 90% of what beats a team, good or bad, is not having the proper equipment to do a job.
As for upgrades and mod, well i tend to use Kevlar, and happy landings, I don't like clotting agent as i find it tends to get me killed more than save me. I primarily fight close or long range with medium range covered by friends, Kevlar excels at both those ranges as it gives you that little extra boost to close the gap in CQC (Close Quarter Combat) and it gives you a few extra hits when dueling other snipers or just taking heat.
Mods for weapons are surprisingly easy, most the mod you unlock as you rank up in whichever role you're currently filling is also generally the best mod for those weapons. Heavy Barrel, unlocked through snipers is an example of a good mod for rifleman as well however, and Cooling Jacket, is generally ill advised on a shotgun.
When it comes to "Blue" weapon mods, such as magazine pull, extended mag, and bandolier. Those really depend on your preference. If your like me at have a serious habit of always reloading after a kill, trying to get it in before my enemy so magazine pull is great for me. However some weapons just benefit far greater from extended mag (Like the Auto-Shotgun) Bandolier is great for Obeya's and OBIR's as they have small clip capacities and snipers are generally far away from teammates so they cant rely on them for a resupply.
There, those are my best tips. Oh and one final thing. Know your role, your weapon, but more importantly know your enemies role and their weapon. If a Gold Star 'forcer is coming at you with a tricked out OCA, and your rocking an OCA too don't assume you can win, he has a serious weapon advantage and judging from his ranks hes probably damn good with it, leave him to your teammates, funnel him out of his element, an OCA wont kill a Obeya user and 100 meters no matter how good they are. Use each member of your team accordingly.
And dot be afraid to take the advantage, sometimes running an extra minute to enter a capture zone from behind the enemy snipers or riflemen is worth the time running, flank whenever you think is possible.