This is very straightforward.
1. Do not enter the product key during installation. Choose to skip that process.
2. When you reach the last day of your 30 day trail do the following...
a. Navigate from Start - All Programs - Accessories - Command Prompt
b. Right click command Prompt and choose Run as Administrator
c. Type the command: slmgr -rearm (enter)
d. Reboot
3. Right click My Computer and choose Properties and you will have another 30 days of trial. If these are not shown in the line under Windows Activation then click the Activate Windows now link and you will see them there.
4. You can do this at most 3x hence 120 days in total.