With the DLC "Operation Anchorage", get Chinese stealth suit, or with the DLC "Point Lookout", get the second Pint Sized Slasher Hat.

Now, collect 2 of the following:

Roving Trader Hat
Raider Blastmaster Helmet
Enclave Officer Hat
Hockey Mask

Now wearing the stealth suit you can wear more then one hat, now that you have one of each hat worn, wear the second hat, repeatedly, you will notice the respective skills increase with each activation. Continue this until you have 100% in:

Barter, Big Guns, Energy Weapons, Explosives, Melee Weapons, and Unarmed.

So long as you never take off the hats, the skills remain, this also works for stat hats too.

Even if you are forced to remove your equipment (as the story requires in some cases, you can still reactivate the same glitch, anywhere any time so long as you are carrying both hats when you get your gear back.