HI! I'm about six months in. Highlighting hills and trees is my bane.
Things I've found to work:
Adding liquid white
Placing highlights slightly above the dark "background shape"
Pulling paint out very thin on the pallet and really tapping in the brush
Things that don't work:
Adding paint thinner
Ton of paint on the brush
Things I want to try:
Letting the painting dry then finishing the highlights