1. Like fractions, make sure you start by asking yourself what the rule is for the operation you are going to do.  Example:  When adding decimals you line the place values and decimal point up, but you do not do this for multiplcation.
2. When rounding or comparing decimals and/or fractions where one or more are repeating decimals make sure you carry the repeating numbers out several places more than what you need so you get a good understanding of what you are rounding or comparing.
3. When dividing decimals make sure you move the decimal point in the dividend and the divisor, the number of decimal values in the divisor. Then you can divide the same as if the divisor was a whole number.
4. Remember a decimal is just a fraction with a denominator that is a power of 10.
5. When finding area and circumference of a circle remember that if you keep the Pi symbol you are giving an exact answer.  If you use some value of Pi, say 3.14, your answer is an approximation.
6. A nice way to remember the formulas for the circumference and area of a circle is:  Cherry pie delicious (C=$\pi$d); Apple pie r two (A=$\pi$r²).