Also, you may want to be in a slightly higher gear than you would use at the same speed and on the same section when seated.
dellzeqq's point about stiff-soled shoes ties in nicely with benb's about cleats.
Cleats are not a 100% necessity (and people had been standing on the pedals for decades before they came along) but they make the whole shooting match much easier.
You'll find that you can't do it for long at first - and in my case not at second, third or fourth either....
Give it a go on climbs, initially for a few rotations of the pedals and then slightly more as it becomes more natural.