[This is part of Shawn Blanc’s blast from the past idea where you link to an old post that is still great.]
Merlin Mann giving you advice to interleave your email replies back in 2005, actually this entire post has nothing but excellent email tips. Merlin:
Make it easy to quote – Power email users will quote and respond to specific sections or sentences of your message. You can facilitate this by keeping your paragraphs short, making them easy to slice and dice.