How do I enter DFU mode on an iPhone 3G OS 4.2.1?

I've tried several solutions to enter DFU mode but none worked: - the Home + Sleep combo - the DFU program here

Somebody has an idea?

DFU has always worked as follows:

1. Turn off the phone
2. Hold power and Home for 10 seconds (exactly 10 seconds, use a clock with seconds)
3. Release Power, but keep holding Home
4. After 5-10 seconds you should get the phone detected on the computer with the screen still black.

I've tested this on 2G, 3G, 3Gs and works on all of them fine.

Here's a video on how to do DFU properly: iPhone DFU Mode

Also I just like to note that this procedure must not be mistaken for recovery mode (iTunes cable screen), which is done by:

1. Hold Home
2. Keep holding Home, press power to start the device (1-2 sec is enough)
3. Release power, keep holding Home until you get to the iTunes cable screen