The two big ways to spot tourists (in my experience) is that they`re either wearing much messier looking clothes than average (T-shirts, raggy jeans, etc) - or they`re wearing something that exposes too much skin.
Be careful in those two areas and you should be alright. Despite what a lot of people think (Thanks to pics around the net, etc) everyday Japanese clothing is pretty conservative.
For pretty average looking clothes, check out some digital catalogs for "everyday wear";
http://www.felissimo.co.jp/iedit/v8/catalogue/index.cfm
and
http://dc.bellemaison.jp/dc/141200904_/index.html
Are pretty standard fare.