First, a quick review of how to set up an "E-Mail Me" hyperlink.
<A HREF="mailto:someone@somewhere.net">E-mail Me!</A>

To answer the question, simply add the SUBJECT field to your mailto anchor:

EXAMPLE: (A HREF="mailto:sosmarrons7@aol.com ?subject=my webpage" )

Notice that the ENTIRE set of command fields fall INSIDE the quote marks " " When adding the subject field, I tend to forget and close quotes before adding it, and invariably, this fails. :)

You can also fill in the .cc (Carbon Copy) and .bcc (Blind Carbon Copy) fields as well.

EXAMPLE: (A HREF="mailto:someone@somewhere.net ?cc=second_person@somewhere.net &bcc=third_person@somewhere.net")

Or you can send a message to a comma separated list of people:

EXAMPLE: (A HREF="mailto:someone@somewhere.net ?cc=second_person@somewhere.net, third_person@somewhere.net " )

Notice the comma between second_person and third_person. You can add as many e-mail addresses as you like on either the mailto: line, the .cc line, or the .bcc line as long as each address is separated by a comma.

If you use more than one command for mailto, REMEMBER that the first command field MUST be preceded by a ? and subsequent command fields must be preceded by an &

EXAMPLE (A HREF="mailto:someone@somewhere.net ?cc=second_person@somewhere.net &subject=Corporate memo")

You can even fill in the message it's-self if you want to using the body= field.

EXAMPLE: (A HREF="mailto:someone@somewhere.net ?subject=Corporate Memo &body=To All Employees, Your Fired Clear out your desk and be out by 5:00")

Legal Commands for mailto:
cc - Carbon Copy
bcc - Blind Carbon Copy
subject - Subject
body - The body of the message.