Basically, there are two approaches: (1) frame-by-frame roto, or (2) using multiple passes of keyers to build a matte.
Unless your rose was shot against a bluescreen, the most accurate method would be to hand-animate a mask (roto). No complicated tricks: just start keyframing. This method is the most time-intensive, but it's also the reason why every visual effects studio has at least one roomful of people doing it all day. Still, you might be able to animate a nice mask in the time it takes to read the rest of this post ;)
Your other option is to try a combination of different keying passes, assembling them into a finished matte. The basic workflow is to make a copy of the shot and then process the copy so that it gives you a part of your finished matte. The number of steps/passes varies hugely, depending on the properties of the shot. This workflow best lends itself to a flowgraph-based compositor like Shake, but you can do it in just about any program.
For example, the rose may stand out as a dark object against a white pillow, so you could try pulling a luminance key (filter) to get started. Before applying the key, you might want to apply some contrast to further darken the rose and whiten the pillow, using filters like Contrast or Threshold. If the stem is a clear green, you could do a color key to isolate only the green in the shot, or maybe the red of the rose petals. Keep in mind, though, that there will probably be other elements in the shot that contain those same colors (like the red in human skin). So, you may want to do loose masks around the rose copies (removing other elements like people) before you try to key them. As you can see, this can go on and on. Sometimes this will work in a single step, sometimes it requires many and lots of noodling. But once you have all your parts, comp them together in a layer and use that layer as the source for an Image Mask on your original rose shot.