This is really something that needs to be best figured out on your own then someone else telling you.
My suggestion is to use very large resolution scans. The reason for this is because you will have to shrink down the finished product to make it fit your wallpaper, hiding any imperfections you might have made in the extraction process.
Me personally, I use Photoshop's Quick Mask mode and paint on a selection using the default round brush, then using that selection, I cut the image I want out. Time consuming but it works, and I find it gets very good results.