One way is to use TABLEs. It can get kind of tricky though, because it's tough to know what each user's resolution and/or browser size might be. Here's an example:
<HTML>
<HEAD>
<TITLE>Alignment Example</TITLE>
</HEAD>
<BODY>
<TABLE BORDER="1" CELLPADDING="0" CELLSPACING="0" HEIGHT="100%">
<TR VALIGN="top"><TD HEIGHT="25%">
<!-- Top content goes here -->
<P>This is the top secion (25%) of the page.</P>
</TD></TR>
<TR VALIGN="top"><TD HEIGHT="25%">
<!-- Middle content goes here -->
<P>This is the middle secion (25%) of the page.</P>
</TD></TR>
<TR VALIGN="bottom"><TD HEIGHT="50%">
<!-- Middle content goes here -->
<P>This is the bottom secion (25%) of the page.</P>
</TD></TR>
</TABLE>
</BODY>
</HTML>
A better approach might be to use a frame layout and put the text and/or image you want in its own frame at the bottom of your layout.
Paul