The best passive defence for a Skiff in a belt is “Someone else’s Hulk” - a better target for the attacker.
The best active defence is “not being there” - running away if something concerns you.
I’ve tended to use a balance of levelling out and improving the shield resistances and adding some sheer mass to the shields (shield resistances stack as I recall, so there’s a limit in usefulness there).
I’m an Exhumer - as others have pointed out I already have a 150+ signature radius. I’m easy to scan down. Assuming you can be bothered bringing a scanning ship (clue: The Exhumer is mining in a belt).
There are several things you can do to make life a little safer: don’t warp to zero on a belt. Go to one extreme of the belt or another to mine: anyone warping in is some distance from you and you have chance to respond.
Always pay attention to what is happening - spike in numbers on local: worry. People you have flagged as “low standing” (because they are a threat to you): worry, people appearing on d-scan: worry. People appearing on grid: worry.
And if you aren’t completely comfortable, go somewhere else.
Where: a system with a lot of belts means the attacker has a harder job finding you among all the options. Find somewhere quiet with little passing traffic so you are less likely to draw attention and can see “oddities” quickly.
No ship, however cunningly fitted to wring the last EHP out of the defence, will save you from a competent and determined attack once they engage. Only your situational awareness will prevent such an event starting in the first place.
So, as it is almost completely irrelevant to your survival: the Skiff fit I use:
Skiff
[Skiff, Skiff]
Mining Laser Upgrade II
Mining Laser Upgrade II
Mining Laser Upgrade II
Medium Shield Extender II
Multispectrum Shield Hardener II
Compact EM Shield Hardener
ML-3 Scoped Survey Scanner
Multispectrum Shield Hardener II
Modulated Strip Miner II
Modulated Strip Miner II
Medium Thermal Shield Reinforcer II
Medium Core Defense Field Extender II
Mining Drone II x5
Acolyte II x5
Infiltrator II x5
Scordite Mining Crystal II x2
Veldspar Mining Crystal II x2
Kernite Mining Crystal II x2
Pyroxeres Mining Crystal II x2