when creating a form that spans multiple pages, you need to use hidden form elements to capture the information from the previous page.
for example:
on the first page the form will look like:
<form id="form1" method="post" action="">
First Name
<input type="text" name="FName" id="FName" />
<br />
Last Name
<input type="text" name="LName" id="LName" />
<br />
<input type="submit" name="button" id="button" value="Submit" />
</form>
on the second page, there will be hidden form elements to capture the first name and last name, as well text boxes to enter new information such as billing address:
<form id="form1" method="post" action="">
Address
<input type="text" name="address" id="address" />
<br />
City
<input type="text" name="city" id="city" />
<br />
State
<input type="text" name="state" id="state" />
<br />
Zip Code
<input type="text" name="zip" id="zip" />
<br />
<input name="FName" type="hidden" id="FName" value="<?php echo((isset($_POST["FName"]))?$_POST["FName"]:"") ?>" />
<input name="LName" type="hidden" id="LName" value="<?php echo((isset($_POST["LName"]))?$_POST["LName"]:"") ?>" />
<br />
<input type="submit" name="button" id="button" value="Submit" />
</form>