Zip Lock Power - I use them for everything when it comes to packing up my Christmas decorations. I keep all my ornaments, and all other home decor pieces in ziplock bags. It helps tremendously when I start decorating again, because it is all neatly packed. Love it.
Recycle and Reuse. I know that sometimes it is easier to just throw things the way they are straight to the trash, but it really is worth to recycle your boxes and paper. Also, I am all about reusing, so if you have boxes, wrapping paper, bows, and ribbons that are still in good shape, you can keep them and reuse them next year. Fold and store things neatly.
Christmas Cards - I usually put all the Christmas cards in a big Ziploc bag and display them the following Christmas. When I get a new one from a specific family, I replace the old one. I feel bad throwing them away. I've heard people scanning them and keeping them that way. I haven't tried that and I am not sure I will find the
patience to deal with my ridiculously stupid scanner time to do it in the future.
Donate - in the spirit of holiday giving and of having space in your closet, donate some old clothes for every new piece of clothing you got as a gift. Awesome way to suppress any hoarding tendencies, of which I know nothing about, and a great way to help good causes by donating to goodwill or any other thrift store in your area.
Replace your old media - if you got cds, dvds or books, you can use websites such as paperbackswap.com, cdswap.com and dvdswap.com, and thus make more space by getting rid of your old ones.
Happy New Year!