"Cheer up" = don't be sad
"Keep up the good work" is fine, but only when someone is already working, it's a bit closer to 辛苦你.
I think the best translation is "You can do it!", or "I believe in you!", but it really depends on the context. There's no single expression in English that means 加油, so I suggest just forgetting about that Chinese expression when speaking in English.