Last week we started doing this, where any errands or trips within one neighborhood are bike/walk only. This covers most of the stores we go to, and quite a few of our friends houses. No hiccups so far, though I can foresee a day that we may want to buy something that doesn't fit into the bike trailer or the weather is just too nasty, though we have fit a hose reel in one trip, as well as 3 bags of groceries and a case of soda water in another.
I set the rule pretty much so I could justify the purchase of the bike trailer, seems to be working. I'm hoping I can keep the habit up during winter if I start now. :)