Can not edit build.prop I am trying to edit the lcd density in build.prop in /system but every time after reboot my changes are lost..

I tried to adb pull + edit + adb push in normal and recovery mode.. 
also i tried fileexpert and super manager but nothing worked.

I heard many people saying about root explorer but its not free.. I want to be sure if my problem will be solved with root explorer then i will buy it..

Please help me out guys..

I am using android 2.3.3 on HTC HD2. My Rom is custom one (HyperDroidGBX-v12)
Answering my own question as found the solution.
We can edit build.prop from mobile. I downloaded ES file explorer which is free. then from optin set "root explorer" and set "mount system as read/write" 
Now edit build.prop and save it using ES file explorer's editor then uncheck "root explorer" and "mount system as read/write" for safety..

hope it wil help someone :)