1. wax the shit out of your board. the lighter you are the more this matters.... wax, scrape, repeat - until you have 3-5 wax jobs on the base.... ski racers stack their waxes like this and it makes your shit slippppppppppppery
2. don't turn.... obviously you have to turn your snowboard but flat-basing the board as much as you can is what gains you speed. stay off your edges as much as you can.
3. put rocks in your pockets.... ok, not really - but being that light you're gonna have to ride with perfect technique if you wanna catch your 165lb friends. try to put on muscle.
4. the board you are riding DOES make a difference. base materials, dampening, length, sidecut - all play into how fast the board will be. a short, noodley, or wide board will always be slower than a longer, stiffer board
5. learn to pump your front foot. downhill weight is what makes you go faster.