What are your tips for reducing costs when shipping/mailing stuff?

Mine are...
-save those white/gray poly bags and use them to ship. Not everything needs to be shipped in a box. Like clothes does not need to be boxed unless there is something hard that might snap. Plus, since weight matters, the poly bags are the light weight. I stopped using padded envelopes because sometimes you don't need the padding and it's just extra weight.
-get a scale
-use flat rate shipping if you are shipping something heavy.

Sorta shipping related but if you are flying somewhere, it's pretty cheap to stuff it in your luggage or take an extra piece of luggage. If it fits in your luggage and is under 50lbs, it will cost me only $25. I looked under USPS for a 12"x12" box that weigh 30 lbs and it was $35 to ship and it would take 7-10 days. This was particularly helpful after my out of state baby shower. Especially for the hand me downs that I got that I wanted to take with me.

What are your tips?
Oh, I never ship through UPS or Fedex. It's always so much more expensive than USPS.