Roll up your sleeves.
Get dog-tags.
Shave.
Get a more accurate hat.
Get longer socks.
Get a belt.
Get more accurate shoes.
Paint your headset orange.
Get a pair of pants in a lighter shade of brown.
Get a tighter-fitting shirt.
Get a longer bat, and add the Sandman symbol.
Put your bag over the other shoulder.
Wrap up to the first knuckle of your thumb, but not around any of the other fingers.

and then give me the contact info of that Soldier behind you because that Black Box is kickass.

At the absolute least, yes, roll up your sleeves please. You can safety pin them so they don't come undone or you can glue yours in place with fabri-tack like I did. Works like a charm and doesn't even wash out.

Next tip is to get yourself a pair of dogtags which you can do cheap and easy at a lot of places. I even got mine custom made for something like $8 off ebay.

And as Drill said, flip the bag. Though if it's a messenger bag, I highly recommend getting or making a sling pack. And if you can't, then adjust the strap of the messenger bag to make it sit as close to on your back rather than down at your hip as you possibly can.

I can't actually see your headset too well from that angle and I know the headset can be something of a pain to get remotely accurate if you don't know what you're doing to mod one or make one from scratch. Though honestly it is pretty easy nearly regardless of what headset or mufflers you're using as a base. So give it a shot if you've got the time.

Those are probably the absolute most basic things you can do with relative ease and affordability. After that it starts to get down to nitty-gritty details. Though to be honest, those are the sort of things that make a good Scout cosplay a great Scout cosplay because just about anyone can find a red or blue tshirt and a black cap in their closet.