One of the most important things in combat is flux-fight. You can fire all the heavy guns you got, deal considerable damage to your enemy, get nearly full flux and then get wrecked by full Harpoon salvo that overloads and destroys your ship.
*When refitting, max out the vents first. Guns and hullmods goes after that.
Don't remove vents without a good reason(specifically, a very good one). Vent speed is quite often life-or-death matte(or prevents such situation at the first place).
*Don't set your non-PD main guns to autofire.
AI tends to miss it against frigate or fighters even with max Gunnery Implants, and can't prioritize targets properly. A waste shot costs your flux pool while contributing none to enemy flux in return. If you need to fire more than two main cannon groups at once, toggle autofire(Ctrl + number in combat).
*Invest reasonably in PD.
Point Defense save your ships against missiles even when shield is down. Not only that, amount of flux generated by PD weapon shots is usually smaller than that of the missile hit on shield.