The only facial exercises that will offer the benefits would be working the masseter muscle (that big muscle on the sides of your jawbone). Doing that might give you a more prominent looking jaw but I think the actual jawline enhancement would be minimal, at best.
For starters you can try chewing gum frequently
That works the masseter muscle and I have heard some ENT surgeons recommend that after jaw wiring to rebuild the muscle. But again I don't know how much, if any, improvement it will offer you. I think a lot of it depends on the face type.
There have been a few facial exercises that have come onto the market and promise jawline enhancement, but I don't know anyone that has used them (so who knows if they're waste of money or not).