If you're an avid Gmail user, you know it's all to easy for the context to get lost when replying to a long email. Blogger John Graham-Cumming points out that Gmail has the same handy feature other email programs have where you can select a block of text, hit reply and only the selected text is quoted in the response.
Instead of simply hitting reply to an email, select the block of text you're specifically replying to, and then hit reply. Only the block of text you selected is quoted in the reply email, which cuts down on all the irrelevant text you get in long email chains. Most email programs have this built in, but many people don't take advantage of it.