Get a tailors tape measure and measure your waist, hips, chest without bust, chest with bust, pant leg inseam, pant leg outseam, and neck. Then hit the online stores and compare those measurements to their sizing charts to get the "correct" size. They are approximate though because different brands use different sizes. Once you find a brand you like and figure out your correct size with them, stick with them until you're confident enough to try stuff on at the store. For jeans specifically, I tend to stick with super stretchy ones (in the waist and legs), because it leaves more room for error and I don't end up back in the store for gaining or losing a couple pounds.
Another option are the various crossdresser clothing sites. Some of them have sizing conversion charts between mens and womens.
For shoes, I found an online chart that listed the same shoe size in several countries around the world for both men and women. Don't expect that you're the same size in a pair of sneakers as you are in a pair of boots though. This is another area where you either buy and return until you get the right size, or suck it up and head to the store for a proper sizing.
Everybody says it and you'll never trust it until you're ready, but most sales people really are only interested in the sale. If you're not passing too well, you may have to be explicit about getting cross gender stuff, but they really couldn't care less as long as they get paid. I thought I passed very well and I had to correct a few myself, so don't take it too badly if it happens. If they do have a problem with it and insist on showing you the wrong stuff, walk out and give the sale to someone else. Maybe one day their pockets will be empty enough that they see the error in their ways.
