All other methods I've found online suggest using a candle or lighter to heat up the brass, which will partially melt the plastic. However, common sense says that leftover gun powder (which is highly flammable) that may be left over in an expired shell probably shouldn't be exposed to an open flame, unless you really desire to be blown to pieces. I personally don't, so I've devised a way that makes it somewhat safer. I don't take any responsibility if you're a dumba$$, though.
Spent shotgun shells
Knitting needle/chopstick or similar tool
2 sets of pliers
Step 1: Remove the Primer
Using a shotgun shell reloader, a hammer and nail, or any other method you choose, remove the shotgun primer.
Step 2: Clean the Shells
Take a piece of scrap fabric or a rag. Carefully wipe down the outside of each shell and then wrap the rag around something like a chopstick or knitting needle or similar tool to really clean inside the shell. Be careful to clean out any remaining gun powder that may still be in there.
Step 3: Wash Your Hands
Get any gun powder off your hands, clothes, and work area.
Step 4: Set Up Your Iron
Set up your iron in some way that will securely hold it with the flat side up--I used a vice grip. Put your iron on a medium high/high setting.
Step 5: Start Melting the Shotgun Shells
Put your clean shotgun shells on the iron (brass side down). You can start by doing one or two at a time, then work up to doing as many as will fit on the iron. The length of time you leave it on the iron depends on several factors:
--The age of your iron
--The brand/gauge of shells
--What material the wad is made out of (plastic ones melt easier than ones that have felt and cork)
I have an old iron, I used a lot of Winchester 12g shells that all had plastic wads, and I filled up my iron and left them on there for about 2 minutes.
Be smart--don't walk away and forget what you're doing and be careful when kids and pets are around
Step 6: Remove the Brass
Take a shell off the iron and, using one of the pliers, grab it a little bit above the brass. Take the other set of pliers and grab it right on the brass. Twist in opposite directions a few times and it should pop right off. If it's not turning, put it back on the iron for a few more seconds. The brass is now separated from the plastic.