Please bear with me while I explain the process for accessing a message draft that you just started, but had to save in order to check something else in the Mail app on your iPhone, iPod Touch, or iPad. Take a deep breath.
First, assuming you have more than one e-mail account (I have four on my iPhone) and you are starting in the "All Inboxes" window, you'll have to tap "Mailboxes" in the upper-left corner. Then, scroll down to the Accounts section and select the proper account for your draft. Now, tap the "Drafts" mailbox. Finally, tap on the draft itself and you're ready to edit.
To get back to "All Inboxes" you will have to back out of the specific account you are in (two taps), then select "All Inboxes" from the main menu. Tedious.
Of course, you're free to do this and it will work. Or, you could do it in one tap, as explained by Obama Pacman.
When you have to cancel a message and you save the draft, iOS Mail creates a shortcut to that draft. To access the shortcut, simply tap and hold the "Compose Message" button (the box and pencil icon at the bottom-right of the Mail app). You'll be taken directly to your last-saved draft.
Keep in mind that the draft shortcut will remain until either the device is restarted or you start a new message and save it as a draft. The shortcut is only for the last-saved draft.
Do you have any slick, time-saving iOS hints or tips? Let me know in the comments!