I just usually start without thinking too much. I've often also thought beforehand of what kind of end result to expect, but I avoid it these days as expectations and reality rarely go hand in hand. For a fairly big perfectionist like myself, it's really demotivating to not meet your own expectations, so it's nicer to jump in with none.
Generally if I just start something, it will develop in my head by itself as I go along, the end result often being somewhat of a suprise even to myself. It's really the curiosity to see in what way something develops that drives me.
Another thing that can really help out is simply to take a break from the whole game. Let ideas brew in your mind for a few days or even weeks, and get back to work. The nicest thing about taking a break is that you don't actually get worse from it, but actually might improve.
And yeah, I agree with Sleven on that motivation really can't be forced. If you don't feel like building, don't build.
...aaaaaand I disagree with Darksaber's claim that building with friends is bad. Sure, there might be striking differences in style if you just jump into it, but after a while building with other people can be extremely fruitful as you can share and develop ideas together, motivate each other to keep building, and eventually develop similar styles.
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