I've, just recently, realized the cure to exactly what ails you Jeremy (having suffered from the same). It's called marriage.

I tell my wife what idea I'm working on. Just this simple act sets in motion a chain of events that tends to make me want to see a project through to the end.

The reason is that, I know she will follow up and ask me later about the project and, if I have abandoned it, will subject me to "The Speech". I hate "The Speech" so much that the entire force of my subconscious and conscious minds will conspire to avoid it - hence shipping products (finally) instead of flitting from one to the other endlessly.

If any of you don't know what "The Speech" is. You just aren't the right audience for this advice....yet. :-)

Note: If you aren't married, or aren't planning to get married due to career. preference, etc. then you should find a friend or family member close enough to you that they will be willing to give you "The Speech". It works.