Yea the best way ive found is to play 3v3 normals, usually late at night. You will find its filled with bots and you can easily S.
On summoners rift, ive found in addition to a decent KDA and CS, also ward as much as you possibly can. This is huge for getting an S. Also doesnt hurt to nab some counter jungle...
Your grade is based on your stats for that champion IN THAT ROLL compared to other summoners. So taking a champ to an off meta lane can be a major major boost.
If you can find someone to help you, waste a game by taking any champ in the jg that wouldnt usually be, and have that person help get you going long enough til you can sustain in the jg by yourself.