If you´d ask most of the experienced travellers for one and only “packing” advice, it would probably be “pack light!”.
Of course every one of us has special needs, if you travel to SE Asia, you´d probably need much less than going to romantic trip to Paris, but still – keep it to minimum.
As you obviously can´t bring your whole closet with you (really, trust me, you can´t:)) you have to pick your essential peaces. I see so many travellers and tourists with overpacked luggage – half of it never to be used. Sometimes avoiding the fees for checked-in baggage isn´t avoidable, but I learned to live from my carry-on most of the time. (Even going for a month, it´s still possible.)
I have a friend who each year travels to Asia for two (or more) months only with his passport, money, insurance documents, toothbrush and spare underwear. He buys the rest there. (This is bit extreme though:)).
There are some great multipurpose accessories like sarong, LBD, vibram fivefingers (ok, that´s not multipurpose, but I love them and you can wear them every day for a month and your feet won´t hurt even after day´s walk), all-in-one makeup products and so on.
You´ll be doing more walking with your backpack on your back, or rolling your luggage behind you than you think. Trust me, you´ll be glad if it isn´t too bulky and heavy. You´ll be much more flexible, spare money and time. (Remember those Check-in lines at the airport?)
Actually, I have to agree with Beth from Wanderlunst and Lipstick about not bringing more than two pairs of shoes. It really can be done, even though it usually needs a bit of practice. (The only time I break this rule is when taking the Vibram FiveFingers with me – since they´re rather special and don´t go with any outfit at all:)).
Still, with few exception I try not to wear things I wouldn´t wear at home in similar situations. (Wearing white sneakers. Or any sneakers for that matter in public. Those terrible looking zip-off trousers. That sort of thing. ) And of course consider the land you´re in – no tank tops in SE Asia, covering your shoulders when visiting a temple, wearing sari (optional) when in India.
Another good tip I´ve got from a friend was to put all things I want to pack for a trip on a bed and then reduce it in a half. Yes, only 50% can make it into my luggage. It always works and sometimes I still find things I never used during the trip.
Since you can have plenty of room in your closet, but less so in your backpack or luggage, the way you fold, stuff and combine clothing should be slightly different. (Wrinkles ladies, think about it!) It´s usually better to roll all your clothes to a tube.
Rolling your garments, skirts, dresses and shirts up prevent most wrinkles (not all, but it´s better than casual folding you use at home). And you save place with this. Use socks and similar small items as “fillers” at the end of the process.
Playing a little “tetris” is the way to go.