Like you said, I think planning is a big factor for motivation. If I know exactly what I want to accomplish with the day then it's easier to get out of bed and get started. If I haven't planned, things tend to drift along in a less productive way.
Another important aspect of planning is setting realistic expectations for the day. I mean if I'm expecting to do a lot of hard work, then somehow I'm more motivated to do it. On the other hand, if I expect the day to be fun or relaxing and then it isn't, I'm less motivated to do my work.
Yes, planning is a good way to help yourself get motivated to see things through. Naturally, I like to plan ahead and know what I'm getting into before I get started. I like to look for means to be more resourceful and make the best of whatever I'm working towards.