Yes, I think we all know the days when you just want to throw on a pair of sweats and be comfortable. However, if you go out in sweats you want to make sure you look great- and not messy or frumpy: here's how.
 
1. I prefer not wearing a matching set: not the sweat pants and sweat shirt. Although that may be maximum comfort, wearing a sweatshirt with jeans or sweatpants with a t-shirt can be more fashionable.
EX: http://media.onsugar.com/files/ons1/192/1922564/32_2009/f27ae6b26fa1866a_kate-bosworth.jpg
Kate Bosworth looks casual and comfortable yet chic with a baggy sweatshirt paired with denim shorts, a purse and sunglasses.
 
2. Try not to be too matchy matchy in another way: color. Unless you are younger, a hot pink top and pants don't look the best.
 
3. Keep makeup and hair chic. Although pulling off a full smokey eye and red lips would be out of place, keeping your hair and makeup neat (such as Kate Bosworth, and Gwen Stefani: http://fashionarchives.files.wordpress.com/2009/04/celebcasualfas.jpg) can update you look and make it fresh.
A messy bun is fine, but really, don't have it messy! A messy bun should look messy in the back but should not have frizz or flyaways you don't want.
 
4. Accessorize well. Pair your outfit with a cute tote bag, sunglasses or necklace. Of course, again, your nicest diamonds (fake or real) don't need to be worn here. However, you can easily pull together a t-shirt and sweatpants with a cute necklace or pull together a sweatshirt and jeans with a pair of cute fun bangles or a statement necklace. A cute tote can pull your look together and sunglasses can too easily. A scarf can also do the trick: but make sure you aren't being swallowed in it. We want to look chic, not frumpy.
 
5. Fit well
Make sure your sweats fit you well. Of course, they shouldn't be skin tight, and should be comfortable: however, you don't want to be drowning in them. You don't want your sleeves longer than your arm and your pants dragging on the floor.
 
6. Don't be cliche
Don't pair your matching Juicy Couture "Juicy" on the butt sweat suit with a Coach bag, Uggs and a messy bun please. If you do go for logo-ed sweatsuit, make sure to keep your accesories neutral: a logo free bag in a neutral color or a leather one, minimal accessories in basic colors. Don't pair a logo sweatsuit with a logo bag and such.
 
7. Shoes
Yes, I have worn sweatpants with Uggs: But don't take it as a fashion statement. It is comfortable and if you tuck them in well it can look OK. Just make sure to keep it looking neat. And please do not wear a sweat suit with Uggs in hot weather. A pair of sneakers or ballet flats are fine.
 
8. Try glowing skin.
With a sweatsuit, you do not want to look frumpy. One good tip is to use a bronzer (try to be natural) or a little blush to give yourself a glowing look to help look the opposite of frumpy: truly chic.
 
9. Be confident.
Yes, although Jessica Simpson (http://cdn2.ioffer.com/img/item/469/292/91/Primp_Women_Ice_Cream_Hoodie_Suits.jpg) is wearing a matching bright blue sweat suit with ice cream cones on it, she does look pretty good I say. She looks confident, smiling, happy. Her shoulders are rolled back and she has a nice expression on her face, and she is glowing. Even with less makeup and not as done hair as hers, being confident and smiling will make you look 10x better than annoyed and frumpy.

http://www.polyvore.com/coming_up/set?id=9208594
I will try to do those
In addition: a requested tip:
What to do on New Years Eve when all your friends are away?

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