when there is x2+a2in the denominator it is advisable to put x = tan@ dx=sec2@
and in case of 1-x/1+x
put x =cos2@
but the more u practise the more u will get it
if u really wanna practise and learn there is thread 4 u
have a look at this 1
http://targetiit.com/iit_jee_forum/posts/integration_siikho_2997.html