Cast Your Underwear to the Wind
Now when I think an article of clothing is on its last legs, I save it in a special travel drawer. Then, when I'm ready to pack, I take as many items as befit the weather at my intended destination.
Underwear is a no-brainer, and you can quote me on that. Let's say your briefs are getting a bit threadbare, or the waistband has lost some elasticity, or there's a hole or two. Save them for your next trip and after you've worn them one last time leave them in your hotel room's trash can.
I do the same with shirts and polos, and occasionally pants. I dumped a whole mess of old clothes in Korea and Cambodia this past winter.
While there are some travelers who like to be extra-fashionable while on vacation, my theory is that where nobody knows me it doesn't much matter what I wear. And it's so nice to have a lighter bag on the way back, or one filled with custom-tailored Madras shirts purchased in Chennai for a song.
My one exception to the rule is socks. I do a lot of walking, and good padded socks and comfortable, sturdy walking shoes are essential.
On my recent cross-country train trip I could have opened the window and literally cast my underwear to the wind. But, not wanting to sully the majestic American landscape with Cherchesian jockey shorts, I ended up doing so only figuratively.