The Right Light
What you have to consider primarily when figuring out how to take a picture of a window reflection is that you are going to need to make the object that is being reflected brighter than the light on the other side of the window. This gives you a few different options, but you should really work with several of them to find the right balance for your photos.
First, wait until it is not excessively bright out the window. This means that the middle of the day should be out, but instead go for early morning or evening. You may not want to wait until night sets in entirely because then the blackness will not display that it is a window clearly. If you want to take a picture of a window reflection then you obviously want both the reflected image and the window to be the focus of the shot rather than just the reflected image. Find a nice middle ground so there is an apparent other side to the window, and this can essentially act as your background image.
The second thing to do is to artificially light your reflection subject, using a higher intensity three point light set up. Since you want to both see them and have their image reflected you will want to conduct the creation of their light set up to be a standard sharp lighting, but understanding that you want them to be lit brighter than they may be otherwise since you will need to bounce their image off of another surface. This is going to be difficult because you do not want to actually reflect direct light off of the window, and you may want to end up dressing them in medium colors so there is no bright glare that comes off of their person onto the reflective surface.
The important point is that when you see the image in the window it stands out as being identifiable and clear. You may want to use a light meter for this to make sure that the spot reading of the reflection is higher than the rest of the window, to make sure that contrast is legitimate.