This year, I came up with the idea, and found time to work on it, early enough to test it out and refine it before Halloween. The idea was Odin, the Norse god.
Some parts of the costume were easy - I bought a wizard's rob and wig/beard - since Odin was the original inspiration for the look of Tolkien's Gandalf - and a pirate's eye patch. The beard that came with the Merlin beard/wig set was way too much, but I came up with a last minute substitution. I've got a pretty good Van Dyke beard going, and it's a little overgrown right now. I combined that with the hair whitening makeup I'd bought for my eyebrows, and it worked pretty well.
For a prop, I wanted to carry Gungnir, Odin's spear, but none of the commercially available spears were appealing. I ended up making it myself, out of cardboard tubes, paper mache and acrylic paint, with some glow in the dark runes added for affect.
One part of the costume never came to fruition, and I might try to add it in the future. I'd like to have Hugin and Munin (Odin's two ravens) props on my shoulders, but I couldn't find anything the right size to carry around.
I think it turned out pretty well. No one at the party I went to had any idea who I was, but I didn't expect any different. If I'd run into any experts on the Norse pantheon, I think they would have figured it out.