I found this to be more of a problem when I was occasionally working from home than now when I'm doing it full time. If it's an unusual situation, it's easy for my wife to think of a bunch of things for me to do. But now that it's an every day thing, she more clearly sees it as "I'm at work" and draws appropriate boundaries. Sure, there's still the random "can you sign for the package I have coming" or "please have the appliance repair person come today." But being able to do that kind of thing is one of the benefits of working from home, and she recognizes that she's asking me to take time away from work for it.

It probably also helps that she works from home with some frequency, so she understands the need to actually be productive at our jobs rather than doing errands and house work all day.