Well, I do wear jeans & sweaters at least twice, sometime 3 times if necessary. Shirts I'll wear at least twice as long as they haven't gotten stained/dirty. Some people are really grossed out by wearing something more than once, but as long as you use deoderant and aren't rolling in dirt, it's perfectly acceptable in my book to wear something more than once. Re-wearing is really at the heart of packing well, with the exception of underwear. (I have heard of guys turing them inside out, but I want no part of that.)
Go light on bulky items, only pack as many jeans/sweaters as necessary.
Is sleepwear necessary? If you can sleep in shorts/t-shirts do so and forgo dedicated sleepwear.
Roll clothes instead of packing them folded. Shirts, especially, compact very nicely rolled.
I carried small bottles of shampoo, etc. Target has small little plastic bottles that I could get several weeks worth of product in, and not have to carry full-size, or even compact size manufacturer bottles.
Go easy on extra shoes. I had my uniform shoes, and ONE other pair (usually leather casual shoes) in my bag.
There's probably more, but I'm being attacked by a toddler demanding a hug, so I'll have to add more later.