why do you want hollow cheeks? i don't know how old you are so i can't offer too much advice. usually facial features are genetic. if you loose a lot of weight you will get hollow cheeks but that isn't healthy. you can use make up to make your face look more slim using shadowing. i think puppy cheeks are cute.
For most people the only way to lose weight in their face is to lose weight all over and unfortunately facial fat is the last to go... Try eating lots of fruits & veggies to get your skin looking great, exercise so your body looks great and play around with some makeup! You can make yourself look almost anyway you want with the right makeup! Smokey eyes draw attention up to the eyes & brows rather than cheeks & lips. Dont wear any BRIGHT cheekcolor it will draw attention to the area. Im sure you look great already Sam, love yourself the way you are! Some of those anorexic looking girls would probably kill for your "cheek fat"!!
Benefit Cosmetics sell a blush/highlighter which when used will give a most sculpted look, or you can try using a deeper shade of blush (to emphasise shadow) and buff the blusher in a line directly below your natural cheek bone to emphase it, but as cheekbones go, most women want the plumper cheeked look its a surgical procedure now to have cheek fillers- be proud of your cheeks whilst others pay thousands to get them and then thousands more to keep them!
Ackshully...there is an exercise to bring definition to the cheeks. Without going into too much detail, because it is copyrighted material from FlexEffect, it's called The Crusher. You pretend you are crushing fruit between your back teeth. That is all.