If you really want to smuggle, first you should build your ships around it, so insulated ship engines and militarized subsystems is a must. Your ships shouldn't be bigger than a cruiser either, but I'd stick to destroyer sized ships at most in order to keep a low sensor profile.
Second, make use of asteroid belts, nebulas, magnetic fields and so on, any terrain that reduces your sensor profile and be in dark mode if you have to sneak around. Strong magnetic fields reduce your profile to 0 no matter how big your fleet is, so you're practically invisible in them and you can lose any pursuers at any distance.
As other said before already, you should also create a distraction if enemy vessels are patrolling a planet, but this also implies that your normal and max burn level must be kept high so you can escape and make use of the diversion.
If you aren't delivering to an enemy faction or your reputation is at least 0 with them, you can go in with your transponder on, then go dark in a belt and do your business. And watch the enemy fleet sensor ranges, don't be caught in them. If the sensor indicator "pings" (when the indicator suddenly catches you out of nowhere then disappears into the distance) that means someone scanned the area and you fell into their sensor range, so you better run at that point.