How you approach this will depend a bit on how closely the octopus will be viewed. If there is a little distance between viewer and prop, and if it's not brightly lit, you'll be fine using the black garbage bag idea. Watcher's suggestion of a big beach ball or humongo balloon for the body is a good one. Pool noodles for the arms are fine - you'll need to taper the ends since that's what real octopus arms do, wrap tightly with black plastic bags, and secure with black duct tape. Suckers can be attached with hot glue.
You might check out some of the threads about giant spiders folks have made and posted here for ideas. The techniques can certainly be adapted to the making of an octopus on the cheap.
I am trying to make a huge octopus, I was going to use pool noodles and ta method from Stiltbeast using garbage bags and a heatgun. The problum I'm having is the noodle melts befor the garbage bag! I am new to this fourm and to the home haunt so if I put this in the wrong place, please forgive
I still intend to use garbage bags but I want the arms to have gurth and be bendable, I don't want to use paper mache because it will not be plyable, The melting of the garbage bags gives it texture and that is what I want. Just cant figure what to put inside that wont melt?
Try running a length of rebar tie wire inside the pool noodles to make them posable, as Lord Homicide suggested. It's dirt cheap, flexible, and holds its shape once bent. You might need to run a double length, but it's inexpensive enough to experiment with to see if it suits your needs.
I had a light bulb moment, I cut tapered lengths of garbage bag and used my seal-a-meal to melt the edges together to form a tube. It worked better than i could hope for, turned it right side out, painted brown on back side then tan on the suction cup side, glued on suction cups and it looks very real