I hear you can get extra wears out of underwears if you're willing to wear them inside out and/or backwards. Hope that helps!!!

Travel Spacebags.

I don't really know what you should pack, but this is how I keep from overpacking:

I write out what I plan/want to pack on slips of paper (e.g. black turtleneck, grey cardigan, red tshirt, pleated skirt, jeans). Then see how many outfits I can mix and match with those slips. If there is anything (that isn't a particular special occasion thing) which I only match up once or twice, I remove it from the pile.

Yeah, underwear can be worn four times before they need washing:

1. Right way round, right way in
2. Right way round, inside-out
3. Back-to front, right way in
4. Back-to-front, inside-out.

It really depends where you're going and what the weather will be but, if you need to take bulky, warm clothing like jackets and can't fit them in, wear them onto the plane, then take them off and stuff them in the overhead lockers.

crush-onastick has a good point - make sure your clothes can mix and match in different ways, particularly shoes, as they are hard to fold. When you put everything in your suitcase, roll them up tightly instead of folding them, which takes up less space and avoids wrinkles (only in the clothes, unfortunately).

Shampoo, shave gel, and the like - just leave them at home, don't even bother with travel sizes, and hit up a drugstore when you get there. Doesn't always work if you absolutely must use one particular posh exclusive hard-to-get brand every day or your face will explode in a mass of boils and you shall simply have to murder yourself, though.

Wear your heaviest pair of shoes on the plane.

Check out Rick Steves, the travel god. He has a great article on packing light.

This is my packing list. Generally wear your heaviest stuff (in many layers) on the plane. Tuck pajamas and other light stuff into the suit bag. If you're going someplace warmer than here, try to not bring/wear too many cold weather clothes.

If you have the ability to do laundry, then just approach it as if you are packing for a week.
Something I do that seems to help is decide shoes first, and decide if I'll be wearing blacks or browns as my neutrals. For my most recent trip, I chose black shoes first and then brought only shirts that would match black + jeans.

Have a couple good over-shirt options if you are going somewhere cold (sweater, blazer, cardigan) that will make it seem like you are varying your outfits more than you are.
Don't bring cotton undershirts and stuff that will stretch out and feel old after one wear. Bring materials that snap back into place after you wear them so that you can re-use them.
Bring one pair of fancy pants, a button-down shirt, and one pair of fancy shoes because you never know where you might end up and if you have to officiate a wedding or something, you want to be prepared.

bring SEVEN CHICKENS

My trick is

1) Only one pair of extra shoes and it should be the opposite of what I'm wearing on the plane. If I travel in high heels, I pack sneakers.
2) Lots of those combed cotton underwear because they tend to pack smaller than say, my BF's big sheet-cotton boxers. Alays pack at least eight or nine, aiming for a once a week wash. You'll need backup and the extra two are that.
3) T-shirts pack small, more T-shirts than sweaters, wear the heaviest sweater while flying (you can take you coat off at the airport.
4) Slacks fold. Dresses fold. Suitjackets take up a lot of space only bring one if you need suit. Shirts fold. All these go in one compartment of the suitcase with the roll-up things like tee's and socks in the other, this way they wrinkle minimally.

Always leave plenty of space when you leave. It's amazing how many times you find the must-have shirt or skirt when you travel. You need to fit them on your return trip.

For winter travel bring the versatile heavy coat. If you expect to be outside a lot, take the heavier one. Being cold is a bitch. Even if it isn't the prettier coat.

Also, crush-onastick's list making is ideal.