Have you read the GMAT club mathbook?

gmat-math-book-87417.htmlI'm just a GMAT novice but I'd recommend to start memorizing.

1) Faculty from 0! to 6!

2) Multiplication with same integers, from 10*10 to 25*25 or so

3) Square roots approximation, sqrt(2), sqrt(3) and so on

4) Triangels, 30,60, 90 triangels = x, 2x and xsqrt(3),

5) Fractions, 1/2 = .5, 1/3 = .3333...infinity 1/4 1/5 1/6 1/7 to 1/12 or so

Various tricks for dividing numbers, sum of digits will tell if 3 is a divisor and so on.

There's plenty of guides

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12/2010 GMATPrep 1 620 (Q34/V41)

01/2011 GMATPrep 2 640 (Q42/V36)

01/2011 GMATPrep 3 700 (Q47/V39)

02/2011 GMATPrep 4 710 (Q48/V39)

02/2011 MGMAT CAT 1 650 (Q46/V32)

02/2011 MGMAT CAT 2 680 (Q46/V36)

02/2011 MGMAT CAT 3 710 (Q45/V41)