I have seen a few guides and some threads on this where ppl have been having trouble getting into DFU mode. This forum has helped me a lot so I thought I'd share my way of getting into DFU mode and it works every time for me. Hope it works for you.
- Have iphone connected to computer
- turn off iphone
- press and hold the home button
- press and hold the power button
- after a few seconds (4-5 is fine), let go of the power button but keep holding down the home button
- after you hear the device connected chime then you can let go of the home button
- you should be in DFU mode now
NOTE: Your screen does not light up at any time if you've successfully got into DFU mode. If it does, you are in Recovery mode.