Have you ever had a friend that gives you REALLY Bad Gifts?
Emphasize the positives.
There's something nice about almost any present - even that sweater with the neon stripes. Talk about how brilliant the colors are or how that god awful vase is truly a one-of-a-kind treasure. You'll be speaking the truth - without being rude or ungracious.
Don't re-gift it.
Someone took the time to choose it for you and they may have put their heart and soul into it. Besides, if it's really that awful, who are you going to give it to? Your worst enemy? Re-gifting almost always comes back to haunt you when your friend discovers you no longer have the "perfect gift" she chose for you. Hide it in a safe spot in the back of your closet that never sees the light of day - and pull it out only when your friend's coming over.
Maybe you can exchange it.
If your dearest friend buys a pair of pants or a skirt for you that you wouldn't be caught dead in, take the item home with you. Call her later and sweetly tell her how beautiful it is, but you've recently gained weight and it doesn't quite fit. Ask if it can be exchanged for something else at the store.
Practice gratitude.
Someone took the time to give you a gift. Relish in the fact that someone loves you enough to give you something. Smile and warmly say "thank you". After all, don't you already have everything you really need?