Assuming you are using InfoPath 2007 / SharePoint 2007, you will need to
publish each form template as a site content type and then add each site
content type to your document library. Each content type must share the same
site columns.

Here are the steps:
1. If you don't already have one, you'll need to create a separate document
library in which to publish the form templates (as opposed to the document
library that stores the submitted forms).
2. In Site Settings, create a site column for each property that will be
shared between the forms. You will probably want to assign a unique group
name to each column so you can find them easily.
3. Create a new site content type derived from Form that contains each of
these site columns.
4. Publish each form template as a new site content type, selecting the
content type you created in step #2 as its parent content type. Use the
"friendly" name of the form as the name of the new content type.
5. When you get to the Property Promotion screen, add the first column,
select the proper element from the form template, select the appropriate site
column, then click OK. Repeat for any additional columns.
6. On the Settings page for your document library:
  a. Delete any custom columns that have already been added
  b. On the Advanced settings page, enable the display of content types.
  c. Add each of the site content types for your forms to the document
library.

After performing these steps, the New button should allow you to select
which form to create, and each saved form should display the promoted columns
in the same fields.