Here are some key formulae it helps to memorise. Admittedly, some are in the formula book, but if you can recall them in your head instantly it saves you the 10 seconds of flipping through the book; it may not seem like much, but those 10 seconds can add up to a couple of extra minutes checking through your work at the end to make sure you pick up all the marks.
Note: Since you can't get the triple line congruence symbol, I am using "==" instead. Consider it a super-equals :P
Definitions of Cosec, Sec, and Cot
Cosec[x] == 1/(sin[x]) , undefined for values of x for which sin[x] = 0
Sec[x] == 1/(cos[x]) , undefined for values of x for which cos[x] = 0