I would like the answer to that question as well... Although I have this problem with my personal projects (that are supposed to be "fun"), not so much professionally.

The problem is that when I get bored with (which might happen very soon, as in, after a few days), it stops being fun... so then your choices are to go on anyway, in which case you're doing drudge work instead of something enjoyable, or move on to a different project, which will likely meet the same fate.

I haven't found a solution for this yet, although I suspect that it might help to have a few other people working on the project as well.