Here are 3 cool tips I just found:
1. Round Off to a Quick Approximation
First determine a level of accuracy you are happy with, then round the numbers appropriately before making the calculation. This is also useful to determine a quick ballpark figure before doing the actual calculation.
2. Break Up Into Parts
Say you have to multiply 613 by three. Try multiplying first 600 by 3, then 10 by 3 then 3 by 3 and adding the results.
4. Quickly Find Three Common Percentages
20% -> Divide the number by 5
33.3% -> Divide the number by 3
50% -> Divide the number by 2
Source