I agree with Ross that trying to take pictures on a windowsill is usually very disappointing. I, too, take mine in an area where I can set things up so that I get decent pictures. I would suggest using a black or dark background (you'll also need something to hold it in place, perhaps a bit of string and some clothespins). I would suggest some good lighting source as well. You don't need anything expensive - a couple of incandescent bulbs or floods in the aluminum clip-on shades will do. I would also suggest a tripod - even a cheap one will get you better pictures. Take several exposures to make sure you get what you want. I've attached a picture of a white phalaenopsis I took recently using a very basic set-up something like I've described. Indeed, I've taken all my pictures using a similar set-up, even when I was photographer for the Southwest Regional Judging Center. If you want to see some of the pictures I've taken using this kind of a set-up you can find a number of them at
Picasa Web Albums - Ron.