You aren't alone. If I had access to the book for my Project Management class, then I could give you exact stats for how terrible most estimates prove to be. The numbers are astounding (the studies are dated, but still adequately horrifying).
Even doubling your estimates is unlikely to have any real reflection on reality.
How far do you want to go though? You could look up COCOMO-II, I saw someone mention function-point analysis, then you could do monte carlo simulation...
The best way is to compare the task to previous similar tasks, but the trick is making sure that tasks are really similar enough to make an accurate comparison.
Even doubling your estimates is unlikely to have any real reflection on reality.
One standard methodology is called three-point estimation: http://en.wikipedia.org/wiki/Three-point_estimation
How far do you want to go though? You could look up COCOMO-II, I saw someone mention function-point analysis, then you could do monte carlo simulation...
The best way is to compare the task to previous similar tasks, but the trick is making sure that tasks are really similar enough to make an accurate comparison.