Cross your arms and legs, yes, but also arch your back so you only contact with your heels and shoulder blades- you get a lot more speed that way. 
^This guy knows.

We have a park nearby my brother and our families go to each summer. It has these:
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loginOn the one on the left, utilizing the method miles describes, my brother has pictures of my in the air over the second hill. Not much air, but air none-the-less. I think they are the favorite slide for my brother and I.