Matter

Read the units in the options before computing anything: they tell you which system to work in, and on a question that offers several expressions they decide the answer on their own. With no calculator, every conversion avoided is time saved and one slip fewer.

Trigger: options carrying units, or a question that lists several quotients and asks which of them give a stated quantity.

Instances:
(i) options in ${\text{g cm}}^{-3}$: stay in grams and centimetres from start to finish, never convert to SI and back;
(ii) options in pascals: convert first, since $\rho gh$ fed with ${\text{g cm}}^{-3}$ and centimetres does not produce pascals;
(iii) mixed-looking units such as ${\text{N cm}}^{-2}$ with ${\text{cm}}^2$ are consistent as given, so use them as given and only bring in $g$ at the last step;
(iv) a list of expressions: reduce each to base units and keep only those whose dimensions match the quantity asked, because the examiner usually arranges for every option to carry the same number.