welcome to the club! if you are going to go hunting here are some of the things you might need...
like you said..sling
and since you live in maine, with lots of wilderness to lost in, a gps.
a "just in case" knife.
an extra T-shirt is essential early in the season,
when you get hot and sweaty, it's nice to put a light shirt on after a hunt.
extra dry socks as well.
a flashlight is nice to see what your doin before it get's light out.
a large cooler with plenty of ice, for your drinks and or the game you are hunting to keep it cool.
plenty of water!
small first aid kit.
possibly rain gear.
after the hunt you will surely need a cleaning kit for the gun. a clean gun is a happy gun.
depending on what you like, a scope maybe?
you can also carry extra ammo in a pouch on your belt so if you do have shell holders on your stock you can keep them in there instead, when you're hunting the brass might shine in the sunlight and whatever you are hunting could see it and get spooked. target shooting is fine to have them on the stock. i know when i hunt turkey, my shells are in the pouch and 3 in the tube. (turkey hunting law)
they have great vision. so i don't take any chances.
i'm sure there is more to say, but i'll let others chime in...
and congrats on the 500!! post up some pics when you have a chance. we all love to drool over them here....