First it is important how you are preparing??
Dont take the wrong train.
if you are alone, prepare your arguments in writing, and then speak aloude.
also prepare anti-arguments and compare, think multidimentions.
it is better if you join a group of people who are working in the same direction i.e group discussion.
Where as Spoken Language is concerned, it can only be improved in "Environment", good teacher plus good companions, where you may have an oppourtunity to speak and convince others. present your arguments with logics.
take it seriously,
Best of Luck.