Just make sure that you set them free a good distance from your home, as they tend to make their way back.. at least a couple of miles, in a nice park or field somewhere..
I'm sorry I have to say this, because you're all obviously well-meaning - but don't you think that it's cruel to set a HOUSE MOUSE free in a park or field? House mice and field mice are two different animals. One lives in a field or park, the other lives in houses and barns. If you have House Mice, and don't want a re-infestation then just kill it quickly and mercifully, but fer gawd's sake don't set it free in a place that is so different to it's natural habitat... that's ridiculous. *mutters* Animal rights my arse