Mental Mathmatics-Calculation

C3H5N3O9

I've long been interested in mental arithmatic/calculation. ie. Doing math in your head. I've come across many different sites, and in turn, ways to do many types of calculations. But I was wondering which, if any, are viewed as better than others.

So, are there any people here who do competitions in this area, etc.? Which approaches are favored? Does anyone know of any sites that are viewed as better than others? Now, at this point, I don't think it's worth spending 5 hours on one approach that in the long run is a 10% better than something that can be learned in 10 minutes. I don't plan on entering in any contests, I just enjoy seeing what I can compute mentally. I have found a decent thing on calendar counting that I had become quite fast at in the past, but over the years forgot the method.

This is greatly appreciated!