How to completely remove data from your phone's internal memory?

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http://forum.xda-developers.com – I have OPTIMUS 2x and if I plan to sell it, I want to remove my personal data from it. I know that external SD cards can be formatted with different (repeating format and data overwriting techniques) but my phone has internal 16gb which has some personal data in it. I can format it but the data will still be there? How can I completely remove my data without effecting the system data. I have a custom rom installed and cwm as well. rooted (HowTos)