So, the questions I ask myself when moving something to/from my SM list are "is this something I want to move on this week?" and "is it urgent/important enough to commit to doing something with it this week?" If the answer to one (or both) of these questions is "no", then it goes on the SM list.
I have very similar questions, but first the background...

When I was first "coached" in methodology like this, I had an inventory of 180 projects. Yup, 180 "things" that I'd said I was going to do (and, I was a high school teacher!). Within a year, the someday/maybe list grew in to the hundreds.

To this day, I still have outstanding projects, areas of focus, and someday/maybes... and weekly I ask myself:

"IS there an action I could take this week to get any more experience in this area?"
"IS this something I'm still even interested in?"
"HAVE I reviewed this enough times in my weekly review to now let it go?"

I'm one of those guys who can open the newspaper and find 4 more people I'd like to write a letter to. So, having a way to manage the "have to," "could do," and "might do" lists is crucial to my health, and quality of life balance.