Make an oval flat track, such as a feild or equivalent, and practice riding it. Stand up on the straight aways and see how far you can lean in the corners. I do this and it helps alot. Its kinda fun racing yourself because you dont get discouraged because all you have to worry about is yourself and having fun. You could also make some choppy sections and practice standing up in them. You'll definitly noctice how much more faster you go when you stand than when you sit. Just have fun and you'll get better.
Try riding a pushbike first standing up and getting used to the balance
Then put your bike on the stand or get someone to hold it and practice standing up and getting a feel for standing up and position and controls while you arent moving
then find a really big paddock or long stretch and try standing up, work your way up, you will get there!
Its me again if you do not remeber I am mgrgirl's boyfriend and she did some rideing at a trial that I was competing at and she did pretty darn good (she did not compete but we rode together) and she has the hang of the controls pretty well for only rideing 3 times in her life so far. I do not push her I want her to decide how much she wants to ride, but If she would just stand up, I know she will learn faster, So thank you ladies I know she (and I) can count on you for tips.
Hopefully I can get her to compete in a trial in the womens class (Junior A lines) I have a 74 TY250 that she rode. I geared it WAY down 9 front and 62 in the rear.