How to get Post variable after reloading the page?

Hi all,
I use java script to dynamically change the form based on the selection of category it shows the sub-category in the another select box , in same form…

But from previous page, i use one variable passes into this current page using post method… that variable is not getting in this page after re-loading the page(i.e after selecting the category,the form reloads to get sub-categories) but before reloading the page,it is coming…
(I used the sessions also but the problem is same)
What is the problem and give me suggestions regarding this…

Thanking you…