For whatever reason, I just really, really like learning languages. When this isn't enough, though, I remember the reason I am learning the language in the first place. I'm learning German because I descend from there, and I really feel tied to that culture. I'm learning Lithuanian for the same reason. This is usually enough to keep me going. Another quick motivational fix for me is to listen to music and singing in my target language. Hearing a language used in such a beautiful way almost immediately motivates me.