To be more specific, I'm not putting the wire in the wall, but rather running it alongside the wall for now until I can get around to actually putting it in the wall.

So I just need something that is capable of holding up 12AWG speaker wire for a run of about 15-20 feet and isn't going to start sagging or anything and will at least look nice. The wall is standard drywall.

Also does anyone make a sort of designer speaker wire, one that might blend in a little better than the usual clear jackets used on wire? I've been buying from Monoprice but they don't offer anything like this.

http://cableorganizer.com/cable-raceway/

simple wood moulding is a LOT cheaper, you can get a few lengths at the Home Depot Lowes or any lumber yard for pennies a foot.
I used a simple L shaped 1 inch. But you can get fancy if you like.
Paint as you like.

Run it behind the baseboards. You can even get custom baseboards that have channels pre-routed.

Also does anyone make a sort of designer speaker wire, one that might blend in a little better than the usual clear jackets used on wire? I've been buying from Monoprice but they don't offer anything like this.

Not sure about designer wire, but they make flat speaker wire, usually for under carpet use.

I use wire mold myself, but 12g is pretty thick, so you might want to test it to make sure it'll actually go through the thinner mold.

I've used white self-adhesive on-wall conduit for 12 guage. Go to a hardware store and take some 12 gauge with you to make sure it fits.