Originally Posted by Flatballer

Generally too much pressure on your hands, which causes you to tense your shoulders and neck, which makes it hurt.

The simplest way to fix this is to move your saddle back. It's necessary to readjust your reach and your saddle height when you do this. The next way is to get stronger and your core and back.

The way it's been explained to me by fitters is to think about what happens when you're standing straight up and then try to sit down. Your butt goes back and your upper body goes forward to maintain your balance. What you're trying to do is lean forward without moving your butt back, so you're off balance, and using your upper body to hold yourself up.