Its difficult to remember each and every formula in IS 456:2000.
As a civil engineer you are supposed to know some things when you work at site.
Go through the entire code for once and focus more on these areas.
Nominal cover to meet durability requirements:
Not less than
20 mm for mild exposure.
30 mm for moderate exposure.
45 mm for severe exposure.
50 mm for very severe exposure.
75 mm for extreme exposure.
Reference: Table 16 of IS 456:2000
Go through Table 16 A of IS 456:2000 also. (For fire resistance).
When you start working at site, the cover that needs to be provided will be main concern. (This is from my personal experience)
Know some properties of materials as per the code.
The nominal maximum size of course aggregates should not be greater that 1/4th of the thickness of member. (Refer to the clause 5.3.3 of IS 456:2000 for more details.)
Modulus of elasticity of steel Es can be taken as 200Kn/m2 (Refer to the clause 5.6.3 of IS 456:2000 for more details.)
Modulus of elasticity of concrete Ec = 5000 √fck
where fck is Characteristic compressive strength of concrete in 28 days.
(Refer to the clause 6.2.1 of IS 456:2000 for more details.)
Unless more accurate calculations are warranted, the unit weights of plain concrete and reinforced concrete made with sand and gravel or crushed natural stone aggregate may be taken as 24 kN/m3 and 25 kN/m3 respectively.
(Refer to the clause 19.2 of IS 456:2000 for more details.)
Span to Depth ratio of beams are very important.
(Refer to the clause 23.2.1 of IS 456:2000 for more details.)
Basics of Limit stress and working stress design are important. Just go through those clauses once in a while. You’ll at least know where to find the information required.
These all came to my mind after seeing the question. (I referred to IS 456:2000). Will add more soon.
Hope this helps.