Sagar,
Snow scenes require around 2 stops of extra exposure. The meter of the camera is to designed as per a 18% gray card. So anything that is brighter than that would be under exposed by the camera to bring it down to midtone level. Similarly a subject that is darker than mid tone would be over exposed to make it brighter.

The snow is about 2 stops brighter than the mid toned 18% gray. So you are supposed to give extra exposure compensation of about 2 stops.

It is easier in Digital cameras, as you can check the histogram and correct your exposure. Try to get it right in the camera itself. Take your time before "shooting". During my formative years, I used to shoot like crazy when I am infront of a good scenery. Later on, I used to find that there were either unwanted elements in the image or there the composition was not upto my taste. So better to take your time before clicking.

In most of the photography competitions, cloning (or popularly known as photoshopping) is not allowed. You are only allowed to do minor colour, contrast and levels corrections. Anything beyond that, becomes a digital illustration, rather than a photo.

Further, resorting to cloning and digital manipulations doesn't help you in becoming a better photographer. Ofcourse, one can become a digital artist, but that is not the focus of this forum.

Please look at the link regarding BBC Wildlife Photographer of the Year competition. One can learn a lot. It is interesting reading as well.

Cheers,
Sabyasachi