@Sunny More,
Many programs save something somewhere that shows the verified date they were installed. Autodesk used to install it on a hidden part of the hard disk. Simply deleting and reinstalling the program doesn't delete this hidden file. I believe Autodesk actually kept it in the MBR of the hard drive so it wasn't accessible for normal users to delete.
One way to get around this is to completely wipe your hard drive, reformat, reinstall your operating system then reinstall the trial program. Of course the means you are reinstalling all the programs every 30 days. An easier way might be to simply buy the program if it has value for you.
That is assuming that the program isn't calling home to a database that shows which computers have installed the trial version and should no longer be able to run it. It is possible for someone to create a unique identifier based on your computer hardware that would allow them to always identify your computer even if you wipe your hard drive. In that case, no matter what you do, you will never be able to run the software again on your existing computer until you buy it.
Most Microsoft products have a way to extend the 30 day trial another 30 days. This is to assist installation across a large enterprise. Once again, they eventually run out and you would not be able to simply delete and reinstall.
Without knowing which software you are using and what system the legal manufacturer uses to identify the end of the trial date there is no way to tell you how to try to steal the program. In most cases it is simply easier to buy the program or find a cheaper product that does the same thing.