I've linked all of my favorite products above since I buy almost everything online these days but you could easily find all of these items at a mix of your local stores.
Poster Tack | its not just for dorm rooms anymore. I use it to hang Christmas cards in pretty rows on the wall but its great to hang all sorts of light decorations quickly without damaging walls.
Fishing Line | I go through so much of this. It's cheap and strong and nearly invisible. We use to hang holiday artwork, string up green garlands and make glass ornaments dangle from the ceiling.
Command Hooks | These little inventions are a decorators best friend. I use them mostly on the ceiling where they are less noticeable and give that air of decorations floating.
Green Floral Wire | If you really want an ornament to stay put, use this instead of a hook. Also great for helping to shape wreathes and garlands. Once its tied in, you never see it again.
Chalkboard Markers | I tend to use these a lot around Christmas time. Not only decorate kraft paper wrapping and write on boxes but also to make DIY signs on painted pieces of wood.
Suction Cups | The easiest way to hang a wreath or anything on mirrors, windows or glass pane doors.
Eye Hooks | I seem to have a never ending use for these - from making advent calendars to artwork to garlands. Buy a big box in a variety of sizes and you'll be covered for so many uses.
Mini Lights | I adore these little battery operated light strings. They are bright and light and can be hung or wrapped around just about anything!
Hope this helps with your decorating to do list. What do you think of my list? Did I miss anything you find helpful?