I think this happens when you use either the nightly.openerp sources and apt-get update for a local installation. Or with any SaaS online version. Why? the code of your source does not include your added field and your code (which is altered now compared to the source) gets overridden with any update.

You can avoid this with two aproaches

1. you can write a separate module for whatever how small change (as such an added field). This will be your own custom module which will not be touched by an update.
2. you do your local installation from launchpad/github sources. When you update such an installation either git or bzr will check which code is newer (the one on the source's side or your local one) and will not change any local code which might be newer then the code from the source (and your new field will be newer then the sources). So it will not be touched

This is my understanding of this matter in theory.  I never had to update my OE 7 yet because it is pretty new, so I have no hands-on experience. If anyone knows it better then me ... I assume my post will be corrected shortly.