If used as wish, I think it can be translated with "Abbi cura di te" or "Riguardati" or even, more generally, "Mi raccomando". It can change in differents contexts, look here for further informations: http://www.wordreference.com/enit/take%20care =)

As a "Goodbye", probably "Stammi bene" will fit the best ("Mi stia bene" is the formally equivalent).