OMG what a fab question and my first thought is: don't fight it when you aren't motivated. We tend to expect ourselves to be on, on, on all the time. But, the truth is, we are human and motivation (like creativity) tends to come in cycles or waves. The 2nd thought that comes up is to make sure you are giving yourself enough consistent downtime to recharge. I guess the root of my answer is: don't stay motivated! Embrace the motivation, give yourself lots of rewards, focus on small, consistent steps, and give yourself permission to not be motivated at all!