Last year I just rode up and down my driveway learning to do a wheelie. You got the right idea; I always stand up, but my dad does them sitting down. I don't use the clutch at all, just pull up on the bars and give it some gas in 2nd gear. Take it slow though, just pulling it up a few inches at a time. Just some practice and you will be able to hold it.
Pulled a wheelie on JasonA's Honda 450 racing of a stoplight last year. It was intentional, but it still kinda scared me.
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