The important parts are a pair of beat up jeans and a jacket. You never seem to see a hunter without a jacket. I just wore a brown tank under my leather jacket. Then add some boots and you're set.
Now the fun part! Props! The Winchesters have no shortage of fun stuff to use when killing monsters.
My boyfriend and I put together an arsenal for pictures.
The important ones are salt (for dispelling ghosts etc), holy water, and a toy gun of some sort. We had a toy handgun that I found for $.90 at goodwill and a toy replica of the Colt. Check out my Supernatural Colt Tutorial. We also had lighters, flasks (filled with holy water), a leather journal, a rosary, a hex bag (made out of a ball of burlap with a hunk of fabric wrapped around it), Dean's amulet (found online for 5 dollars), a ninja star, and very fake IDs if you're so inclined. Put them on your person and you're ready! I tried to talk my boyfriend into carrying a pie, but that didn't go over as well. If we ever do these costumes for a con, there will be pie.
Lastly, you'll need an anti-demon possession tattoo. Google image search and draw it on with sharpie. Now leave your house listening to Carry On My Wayward Son. You'll never feel more like a badass. I promise.
All Done!
Hunters ready for action!