My favorite way to change up my living room decor is to add new pillow covers. It can update the room surprisingly quickly. Here's a pattern for the easiest pillow covers you can imagine. You don't even need a sewing machine, you can do this with a little hand stitching and "stitch witchery".
Firstly, Ikea sells really cheap pillow forms and even has one (IRMA) that's 14"x 14" for 99 cents. This is a great size for a sofa. So I'll base this pattern on a 14x14 pillow.
So, find some kick-ass fabric that you think will be a good addition to your room. My online favs are craftyplanet.com and reprodepot.com. For a 14" pillow you'll need one yard of fabric. Cottons are good, and home dec fabrics work too. You could also use denim, or pre-quilted fabric. For a smaller pillow you can even use a T-Shirt or favorite old sweater!
Cut the fabric in a 36" x 17" rectangle. Lay the fabric out with the "right" side down. Turn the edges 1/4 inch and iron, then turn 1/4 inch again and pin. Take this to your sewing machine and straight stitch across. If you don't have a sewing machine, you can use a strip of "stitch witchery" and when you do the second fold, place it between the fold and the "wrong" side of the fabric where it meets. Iron in place, do this for both ends.
Next, set the fabric down with the "right" side up, facing you. Set the pillow on the fabric to gauge where you want the middle to be. Fold the ends up over the pillow and pin it on one side.
Take the pillow out. Level out the fabric and pin on the other side as well. At this point, the fabric in the middle (where you hemmed) should be overlapping. Take this over to your sewing machine and sew up the edges using a 1/4inch seam allowance. If you don't have a sewing amchine you would hand-sew the sides. It will be stronger this way than if you just use "stitch witchery".
Lastly, take out all of the pins and turn the pillow cover so that the "right" side is facing out. Your cover is ready to use!
Just stuff your pillow in there, and set it on your sofa or chair!