1. The first non-zero digit is the first significant digit.
2. Write the number in scientific notation so that the first non-zero digit is followed by a decimal point.
3. Zeros that have non-zero digit before and after them (like 1.057) are significant.
4. Zeros at the end of the number (“trailing zeros”) are usually insignificant, unless otherwise stated.
5. When numbers are multiplied or divided, the answer should have the same number of significant figures as the number with the least significant figures. For example:
   
   4.3 x 7.863 = 34 = 3.4 x 10¹
6. When numbers are added or subtracted, the answer should have the same number of digits following the decimal as the number with the
   least number of digits following the decimal. (e.g. 4.3 + 7.863 = 12.2)
7. Constants, like the 2 in the equation for the circumference of a circle (c = 2 π r)
   should not be used when determining how many significant figures your answer should have.

For more information and practice, see the review at the Khan Academy