I would advise a combination of three (or maybe four) things:
1) Study reference materials: Screen shots, official artwork, official character sketches and concept art, and even fan art; save off some of the images of your favorite poses to a folder in your smartphone or computer, so you can refer to them later.
2) Study "real" posing guides - ones that aren't cosplay-specific. Different body types have different tricks and pose techniques that are more or less flattering. If you're stick-thin, there are some posing tricks that you can use to give the appearance of curves; if you're pudgy, there are some tricks that you can use to hide/disguise the areas you don't like while showing off the areas that you do like. I typed up a little bit on posing here: http://www.cosplay.com/showthread.ph...87&postcount=7
3) Practice in front of a mirror. Don't forget to practice facial expressions; Yoko has a pretty wide variety, so that should be fun.
4) If you have a willing friend, practice posing and have them take some snapshots. For posing practice, even simple snapshots from a cell phone or point-n-shoot will work, and you don't have to be in full costume.