Mega Download Mode
I have a Galaxy Mega GT-I9205, which I have rooted and installed CWM recovery. I have had enormous problems in getting this phone into download mode with the stated key sequence of pressing Volume down, Home and Power simultaneously. It does work but probably only once in 40 tries, the other 39 times it just starts up normally and then has to be shut down before you can try again
I managed to find a different way to do this that works nearly every time and thought I would share it with you in case anyone else has the same problem, this is what you do.
Get your usb lead and plug it into your charger or a live usb socket on a computer. Plug the micro usb end into the phone and then without delay hold down the home and volume down buttons. Do not touch the power button. Keep these two buttons held down until the download mode warning screen appears (do not be tempted to let the buttons go when the phone vibrates to tell you it is connected to a power source or the method will fail). When you get the warning screen use the volume up button to confirm as per usual.
A couple of things I should say, there is absolutely nothing wrong with any of the buttons on my phone, they work perfectly in normal use, and secondly, I am not really a 'power user' when it comes to phones, I am more at home with computers, so this may be old news to you, but I have never read it before.
Hope this helps someone.