I have set
export PATH=$PATH:/home/user/android-sdk-linux/platform-tools
and execute the above line in command line and later if I type echo $PATH, I could see the path added. But after restarting the system I couldn't see it any more. But putting the system to sleep and then starting again, I could still see it. How to resolve the issue?
Adding it using export on command line would just set it for the current session.To set it permanently you need to add it to .bashrc or /etc/profile.
Your question is similar to: How to permanently export a variable in Linux?