As an interesting counterpoint, I often find myself with the exact opposite problem: I start too often. It's to the point now where I have 4 or 5 active side projects, and then I think of something else cool that I want to build... and then it's hard to actually work on and continue with them, especially after the 'fun stuff' of the initial creation is over.
I feel the same way. I am far too often starting a new project, or coming up with new random ideas. I put things aside and have to work hard at finishing them. I think starting is the easiest part, and finishing is the skill that needs to be preached.
I hear you there. I've created a few sites, and hit a point this summer where I asked myself why I was building yet another site. The one I learned by far the most from was the one I completed and had real customers heading to.
Since then, I've really worked hard to focus on the one "startup" I'm involved with. Sometimes it takes some patience to get to the finish line, but once I've crossed it, I've found it can get interesting again.
Same story here. For those who have required skills staring is easy. But then we either lose motivation or start to think about another cool idea. So, for us the opposite (well not really opposite) should be stated - finish what you have already started.