Thank you for your question. This is actually a fairly common problem. There are a couple of steps you need to take:
1) Try not to apply aftershaves directly to your cheeks or sides of your neck (especially not in the morning). The alcohol and fragrance in aftershave makes the skin sun sensitive and therefore the skin pigments in that area to as a protective mechanism.
2) During the day, make sure you are wearing a good facial SPF every single day as this will prevent the marks from becoming any darker and will slow down the formation of new pigment in the area.
3) To lighten the marks that you already have, I would suggest a good lightening serum that contains pigmentation lighteners and inhibitors, which you can apply at night before you go to bed. Something like Brighter Concentrate from Lamelle, or Lumixyl or if you want a good all round product that contains great ingredients for your skin in general, something like Skin Medica’s Lytera Serum.