Break numbers apart
Place value lets us decompose a calculation into useful pieces. A multiplication problem can become a sum of smaller products; a subtraction problem can be handled by counting up; a percentage can be built from familiar fractions such as one half, one quarter or one tenth.
Use complements
Numbers close to a round value are often easier to handle through complements. Instead of treating 99 as an awkward number, it can be viewed as 100 minus 1. This is not a trick so much as a change in representation.
Estimate before calculating
An estimate creates a quick reasonableness check. If the exact answer lands far outside the expected range, that is a signal to review the work. In practical settings, knowing the approximate scale of an answer can be nearly as important as obtaining the exact value.