www.velowear.com I find have the most reliable sizing charts. I bought jerseys form Performance and Nashbar and sometime the fit was right, sometime not and had to return them. It seems to vary from one brand to another, often even between models.

If you really want a great fit and best material, buy from an LBS. A quality shirt at right size will last you for years. The one I wear the most is a Gore-Tex I purchased three years ago at an LBS, it was a little pricey at ~80.00 but it is much better than anything else I purchased.

Things you need to look for: fit, feel, pockets, visibility, long zip.