There are two extremes when it comes to email formatting: some people prefer strictly plain text, while others like fancy fonts, background graphics, and other kinds of embellishments. No matter where you fall on that spectrum, knowing how to set your personal approach as defaults—and how to override them when necessary for specific messages—means you can easily get your message across in your own inimitable style.
Set text formatting (and cross your fingers)
When it comes to formatting text in a message, you have one major choice to make: plain versus rich text. Plain text is a throwback to alphanumeric-only communications where, for instance, you add **asterisks** or __underscores__ for emphasis instead of using actual italics or boldface. Rich text provides an alternative, using behind-the-scenes HTML coding to format not only text, but also lists.

Even if you don’t use red Comic Sans, the rich text option (bottom) improves readability. You’ll have to hope, however, that your recipient uses rich text, too.
Whether you prefer plain or rich text, you can set your favored default by going to the Composing tab of Mail > Preferences and then choosing Rich Text or Plain Text from the Message Format menu. You can then reverse that choice for a specific message with Format > Make Plain Text/Make Rich Text (Command-Shift-T). Or, if your default is plain text, you can override that just by applying formatting (bold or bulleted, for instance); you’ll be asked if you want to switch to rich text for that message.
If you have rich text enabled (either as the default or a one-off choice), you can italicize your perfect bon mot with a quick Command-I, or lovingly craft a message in bold Comic Sans with a flowing Zapfino signature by using the Font Panel (Format > Show Fonts, Command-T). You can even set Comic Sans as your default message font in Mail > Preferences: in the Fonts & Colors tab, click Select next to the Message Font field and choose your font (which will also be used for received messages—retroactively!).
One other tip for configuring formatting for outgoing messages: If you want to honor your correspondents’ formatting choices, you can match them automatically by going to the Composing pane of Mail Preferences and checking Use the Same Message Format as the Original Message.
Whether your recipient sees your formatting—from simple italic and bold type to regular or multi-level bulleted and numbered lists—depends entirely on her email app’s capability and settings and font availability.
Cure attachment disorder
Let’s say you’re helping a neophyte friend learn how to navigate Finder folders. To illustrate your points, you want to include a few screenshots. You inserted those shots in the right places in your instructions as you wrote, with no apparent problem. But when you tried to add a few more shots later, they all clumped at the bottom of the message window, no matter where you tried to insert them.

A message formatted with interspersed, fully displayed attachments (background) and as icons at the end of the message (foreground).
You can override the default placement of graphics in a message window for those after-the-fact additions by unchecking Insert Attachments at End of Message in the Edit > Attachments submenu. This setting affects the way the message appears to your recipient, too.
Don’t bother looking in Mail’s preferences to make this setting the default for future attachments. Instead, make sure no message window is active and head back to the Edit > Attachments submenu, and uncheck Always Insert Attachments at End of Message..
Whether you put attachments at the end or within a message, sometimes (such as when you’re including step-by-step illustrations), you want to see the actual attachment instead of its icon. But at other times you just want to your attachments to appear as file icons. You can easily control this by Control-clicking on the attachment in the message window; in the subsequent contextual menu, choose View as Icon to change the full-view attachment to an icon, or View in Place for the opposite. Unfortunately, there’s no way of ensuring the recipient will see it the way you sent it (but it’s a fifty-fifty chance).
Your signature look
Your automatic signature—with your company contact information, Twitter handle, blog URL, and so on—is a joy to behold. But when you reply to a message from a client, and that reply includes the original message’s entire contents, your signature is inserted after that quotation, well out of sight without a lot of scrolling.
You can, however, set it to appear after your reply text, before the quoted material, by going to the Signature pane of Mail Preferences. There, to have all your signatures appear before quoted material, click All Signatures in the first column and check Place Signature Above Quoted Text. Alternatively, you can tailor signature behavior for individual email accounts by clicking an account in the first column and a signature in the second before selecting the checkbox. Your signature-placement choice will be seen on the receiving end.