The amount of guidance available about packing is amazing. I personally always try to travel with only a carry on bag and I follow three basic rules:
Rule #1: STOP - When packing stop thinking about your trip as a series of days for which you will need fresh clean clothes. Instead select items that can be mixed and matched, layered, etc. You may not like it, but you might have to wear an item more than once!
Rule #2: DROP - Greatly reduce the amount of things you take with you by simply dropping a third of the items. Once you have laid out all of your items (primarily clothing), take a second look and get rid of about a third of the amount. For underwear and socks pack blended or quick-dry fabrics, so that you can wash and dry them overnight.
Rule#3: ROLL - Save space and avoid wrinkled clothes by rolling items. This How to video by Von Malegowski is very helpful.