Christina the good thing about design is everything is influenced from something else. There is nothing that doesn't take influence or elements from something you've already seen. For me, I always treat my designs as an equation. Ex. concept + theme(idea) = my design. When I start an art project I try to measure out the things I can do, with the time I have available. After that I begin to brainstorm for a while until I come up with a concrete idea and I can make concept statement from it. Recently I've gotten back to making mood boards so I can visually see the direction I want to go.
As far as crossing the line of design I say as long as you don't copy the same thing and turn it in as your own that counts as creativity and inspiration, because the goal of design is to improve or expand an idea or concept that somebody has left before you. Me personally I'm inspired by alot of music. When I start designing I take music that displays the emotion or feeling I'm going for. Also reading or design blogs give me inspiration as well.
