If your dx is running gingerbread you just power off the phone or pull the battery out and put it back in. Then press your home button first then press the power button and hold both buttons in till you see a triangle on the screen. Next you press both volume buttons in at the same time. That will get you in recovery. Next use your volume button to scroll down to wipe cache partition and select it with your power button. Then select yes to wipe cache and once its done select reboot system. That's it.
If your on froyo do the same thing but instead of pressing both volume buttons to get in recovery you'll press your search hardkey button. You'll also use your camera button to select instead of your power button.
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