While helping a Rose of Sharon Cassady, or just Cass' I found that I started losing fame with the NCR because of the actions I helped her commit. Now it isn't that big of a deal, but I'd have like to stay friendly with the NCR having already helped them several times. Is there any way that anyone knows of that I can do to regain their trust?

Once you get far enough in the campaign your reputation with both the Legion and NCR reset to neutral if you are seen negatively by either.