How do I keep a helmet from slipping?

[OrbitalDropRat]

Finished 3d printing my Mk 7 helmet, but it keeps slipping out of place. I have some foam blocks inside of it but it is still not enough. should I try and get a buckle/chin strap or is there a better method other people use?

 

[Jocko03]

Its all a matter of padding. How much you use, where you put it, what kind you use. My fred is held on by the ear muff headphones in it, but my ODST its just strategically placed.

 

[Zevbutt]

Both my Gungnir and Artaius are completely empty on the inside. Instead of padding i got a rugby helmet. put it on, then shoved my head in the helmet. It has held them pretty snug

 

[SgtSaint]

I have some foam blocks inside of it but it is still not enough.

Before saying "just shove more foam in it" let me ask...
How much too big is it, if you have foam blocks but still need more?
Did you make it scaled to fit you? Or is it at whatever size the modeler made it, even if that's way bigger than your head?