Shouldn't you understand what you're doing?
"The trick to flying is to be able to throw yourself at the ground, and miss."
The quote does appear in HHGTG.
[edit: I don't have a copy of the original radio scripts on hand, but from the book:
"The Guide says there is an art to flying," said Ford,
"or rather a knack. The knack lies in learning how to
throw yourself at the ground and miss." He smiled
weakly. He pointed at the knees of his trousers and
held his arms up to show the elbows. They were all
torn and worn through.
"I haven't done very well so far," he said. He stuck
out his hand. "I'm very glad to see you again, Arthur,"
he added.
-- Life, the Universe and Everything