How common is it to work extra with one's professor after getting a M.S.? I'm in a situation where I am continuing to work on some of my M.S. research work after I have filed and will receive my M.S.

It is fine with me and I think it's good because there were loose ends. This is computer science field, and it involves a programming project we want to improve.

In the back of my mind, I'm hoping we can write a journal article. Part of me, though, wonders about whether this may be risky.  Will my professor make excessive demands on me, even though we're officially finished?  I also need to focus on other demands, such as my job. 

So I worry a little about the expectations.

How common is it to continue working after getting one's MS, but without any official agreement (i.e., I just informally meet with my prof and discuss my progress).

Also, in case you're wondering, I'm not currently interested in doing a PhD (maybe later, just not anytime too soon).
Your concern that this is "risky" is somewhat vague, but generally, I'd say that working on something like this is extremely common. I've never "cleanly" finished a project, where it is fully wrapped up, with nothing else to do right as my degree was awarded, I moved to a new position, etc.

I'd say it's extremely common, and if you think that extra work might result in a publication, fairly valuable. There is the possibility of things being too demanding, but generally people are pretty understanding of other obligations that come up, especially if you let them know in advance.