Where

It's probably best to have the place at the entrance of your house, clearly marked. If you don't have space a box will do as well.

How to get stuff

Encourage your guests to leave stuff they want to get rid off and to take stuff they need or they can give as a present to other people (e.g. within hospitality exchange, to their next hosts).

In many (affluent) places of the world it's common to find perfectly good stuff in the street. Take it home if you think someone visiting your house might want it. In case of clothes: put them in the washing machine first.

See also what kinda stuff would be nice.

Mirrors

If you have lots of clothes in your boutique it's nice to have one or more mirrors so people can fit.