I know that this probably has as many answers as users here on SO, but software estimation always seemed like an esoteric science. Software developers don't have a magic book to refer to as exist in many other industries.

I've been spending the last couple of days working on putting together some estimates for a bit of work that I am proposing for a freelance project that I am working on and am having trouble getting it down. I'm not experienced with any real software estimation practices and am trying to go from the gut based on my experience but also trying to be a little loose (not too loose though) on the estimates to leave me a bit of room to work.

I read this blog entry http://blogs.popart.com/2007/07/what-scotty-from-star-trek-can-teach-us-about-managing-expectations/ that was linked to from SO and would like to start tracking my estimates at work as well even though I'm not really required to create estimates there.

What tools or techniques would you recommend? Also, how much padding do you usually add in to a time estimate?