The mistake I make the most often is getting too far ahead of an idea.
I think I've discovered something great, so I share my ideas with the world and begin to build an idea around them. Then I make the fatal mistake and, in the heat of the initial excitement, announce a plan to the world.
I still haven't written a book. I still haven't published a Windows 8 app. The one public goal I've managed to stick with thus far is that of daily blogging. It's been difficult, and has taught me a lot about both setting and keeping an objective.
Firstly, a goal has to be realistically measurable. "I will write a book" might seem measurable, but has little quantifiable outcome. What kind of book? How long? Where and in what format will it be published?
Some could argue that this site - a daily blog with over 300k words on various topics counts as a book. In addition to my writings on software and faith, it even includes the entirety of a NaNoWriMo novel (50k words of creative prose on a single topic).
But my blog today isn't what I set out to create initially. When I announced I would write a book, I had a specific goal in mind - a study on marketing. My public goal was more ambiguous - a hedge against changing my mind on the topic - and thus, an impossible goal to hold me to.
A better objective would be in the form of "I will write, edit, and publish 3D Marketing, a study on storytelling as marketing, by December 31 of this year."
It's clear, concise, and something others can hold me to.[ref]Note, I'm not announcing this book right now. I have other objectives at the moment.[/ref]
Keeping an objective is difficult. Blogging daily isn't something that comes easy - particularly with a busy schedule that includes traveling for work, attending conferences, and the occasional low-tech camping trip. I have to plan articles in advance and carve out time in my schedule for production.
At times my forgetfulness means this time is after midnight because I elected to see a movie with friends or sleep on a plane rather than write. But I always set the time aside; even when it hurts.
Failing to set time aside for an objective means you don't take it seriously. If you don't take your goal seriously, why would anyone else hold you to it? You are the first line of accountability. Stay true to yourself and your goals, and your friends, family, and colleagues will support and encourage you as you work towards them.