I think your best option would be to paint the entire wall green and then paint over in white using your stencil. You'll probably need a few coats of white paint, but I imagine it would still be less of a headache than painting the green areas around the stencil design.

Dont stencil. Paint the room green and use a white wall sticker instead for a similar look that isnt so "permanent"

I would go to a drafting or architect supply store and purchase drafting mylar. It is comes in a variety of thinknesses and is a coated plastic. This coating allows pencil and ink to adhere onto the film. Since this material is semi transparent, it will make copying your image from your original source easier. The plastic itself is strong enough to take the abuse of cutting and moving and repositioning.

rather than stenciling, it might be simpler to just lightly trace your design onto the wall with pencil, then fill in your leaves with painter's tape. go over the outside edge of the tape with a bit of white and let it dry (to prevent the green from bleeding). then you can paint green in between your white leaves.

I'm going to stencil one wall of my bedroom soon, too, and planning to paint he wall the darker shade first, then the negative spaces the lighter color. I was going to use spray adhesive to stick the stencil on the wall while painting. I'll be watching this tread for more tips!

How about drawing your image out on acetate and projecting it onto the wall? Then you can trace your design with a pencil, outline it with your dark green paint and fill in the negative space. This way you don't have to worry about stencils falling off or pain leaking from underneath.

If you take the image to a sign-making company, they can produce a sign for you to use as a resist (stencils are also resists), you remove the portion you don't want and self-stick the other portion to the wall. So ultimately it's your choice whether you use the positive or the negative of the image (or both on different surfaces). Once you've adhered that stencil, and brushed out the bubbles, then paint over the edge where the wall & stencil meet with either the wall color paint - or preferably - artist's glazing medium. Let that dry and it will act as its own resist to prevent the paint from seeping under the edge of the stencil. You will have crisp sharp professional edges on your finished work. What the sign company charges will vary, depending on the size and complexity of your design, but can be as little as $40 and up into the $hundreds. In this economy, you can bargain for the services, ask if it's their 'best offer' and shop around.

I would go the projecting the image onto the wall route. You should be able to rent the projector. The library in the town I used to live in rented them out but I'm sure there are other sources.

This is a good example...
http://jenloveskev.com/2009/06/09/our-new-bedroom/#more-900

I've searched a lot for cool negative/reverse stencils for a project I want to do but really haven't found anything.

Cut leaves out of whatever material you deem appropriate or most useful (testing helps, here). Spray the back with a temporary adhesive. Adhere to wall. Paint. Remove material from walls. voila.

It seems it would be more efficient to paint the wall green first and then stencil in white, but if you're set on stenciling the green you can always trace the stencil you have and make your own for the pattern you want.

I used an overhead projector to trace a vine pattern on one of my dining room walls. It doesn't look as "perfect" as stencils would look, but we think it has a more organic feel with that way. It was really cheap and easy. We painted the walls a dark blue and then used a bronze leafing pen to trace the pattern on the wall. We just traced the outline of the vines, leaves, and flowers and didn't color them in.

Stencils are a PITA - you'er better off just drawing on your leaf shapes and painting the green freehand around them

An alternative could be to get wall paper from Sherwin-Williams (which is great for apartment dwellers, because it's easy to put up and take down!) Trace your stencil onto a color or cut out one of their designs, then stick it to your wall. They also have murals. Not too bad for the amount you pay. http://www.swdecorating.com/

Oh yes and the type of wallpaper I'm referring to is called Easychange.

I am hugely late to this thread and I hope that you got the job done successfully. If not ask me. They were my hands in the picture I stencil many reverse and negative patterns and I could have told you. It is a simple process. Cut a few leaf leaf shapes from Mylar film. Adhere to the wall with a light spray of repision adhesive. Apply the paint to the wall with a roller. Roll away the excess paint so that so that the paint is damp not wet. Start the rollering process on the mylar shapes and roll off them. The edges will be clean if you worked off the excess paint. Remove the leaf mask to reveal a white leaf on your newly painted green wall. This method works really well on patterned wallpaper. There are lots of stencil how to's on my blog. Those that say that stencilling is a pain don't know how to do it correctly.