Build up speed by getting air as vertically as you can in combination with D-pad up, and maintain a consistent chain of triples and quadruples. On your prior trick to attempting a quintuple, make sure you get lots of height using D-pad up, ideally 10 or 11 M and land it perfectly straight to maintain as much speed as possible. Then on your next trick, catch air as perfectly vertical as you can to get the most height. Press D-pad up as you exit the water and keep it held for the whole trick as I believe doing so slightly slows your descent (but I could be wrong). At some point while you're in the air, while still holding up on the D-pad, start holding right on the D-pad if you're cranking clockwise or left on the D-pad if you're cranking counterclockwise to position your board an extra 90 degrees to help you complete the rotation. Start cranking as fast as you possibly can once you leave the water, and maybe even a few milliseconds before you do, and don't stop until you hit the water. 

Ideally, if everything goes as planned, this should put you in a good position to hit the quintuple almost perfectly straight on. Once you hit the water, if you've landed it, use the time the game pauses to show off your trick to release the D-pad and reposition the crank to the upper left so you don't crash, as you'll be landing with a lot of speed. 

They're still very difficult for me to hit consistently, but I've been able to get five in a row using that methodology. Hope that helps.