I personally like to use the factor notation of the prefixes, so when I see "centi"meter, I immediately think of "10^-2"meter.
So, converting cm^3 to m^3 will only require me to substitute (10^-2)^3 = 10^-6 m^3. Also, it helps when you imagine which one is larger, a centimeter or meter to verify that you are heading the right way, so when you are representing a cube with 1cm dimension on each side, it's a small cube with each dimension the width of a finger and I am sure you can imagine how long a meter ruler is. Then obviously, the meter volume will have the smaller superscript (-6) resulting 0.000001 m^3 (which is the same as 1cm^3).
Looking at your example of converting the density:
Remembering that 1g (smaller) = 10^-3kg (larger in quantity, hence smaller superscript)
1.2 g/cm^3 = 1.2 x (10^-3 kg)/ (10^-6 m^3) = 1.2 x 10 ^ [-3 - (-6)] kg/m^3 = 1.2 x 10^3 kg/m^3 = 1200 kg/m^3
It is useful to remember these dimensions in real-life like 1mm is the width of your fingernail and 1 cm is the width of your finger.