An efficient trade is someone offering what you are looking for. In most cases you don't actually transfer miles, as that is usually as expensive as buying miles, and make a trade unwieldy.

You will see a lot of lopsided trades of late. People offering 1/2 CPM value for AA miles, for instance.

Generally, you post what you need, and what you're offering in trade. Some miles aren't as valuable as others. For instance, I'd say 1 AA mile is worth 2 DL miles. Other times, if you really need a particular award, you have to offer more of your miles or points to make the other side willing to trade.