This topic is one of the biggest problems when it comes to being an artist. Finding motivation is hard enough but staying in that state is even harder, to stay motivated it take practice and sacrifice and is not easy.
What can take away from being motivated...anything from friends wanting to hang out, to family, or just life, these are things that can easily distract you and will take time away from you spending time honing your craft, thats not to say that you have control of all these situations particularly life, but trick is rolling with whatever is happening and just pushing through and doing something everyday whether it is a sketch or life drawing, or just quick doodle just as long as your getting your ideas out as much as possible.
A good way to start is making drawing or painting something a habit and part of your daily life, once you get to a point like this it will become easier to make sure that your motivation stays at its highest plus your getting any idea out and after a while idea will come easier and faster.
sources:
Lieu, C. (2013, November 5). Ask the Art Professor: How Do You Stay Motivated After School?. The Huffington Post. Retrieved December 1, 2013, from http://www.huffingtonpost.com/clara-lieu/ask-the-art-professor-how_2_b_4216235.html