get yer ass off the saddle and they'll be ALOT easier. you want to stand up and forward, bend your knees, and basically have your ass in front of your saddle for riding along the top of logs. it will give you better balance and allow you to react to and correct problems more quickly.
i don't have any video of me doing the big log crossing at 6-mile, but i think i was the first, before it was built up with sticks on the high side.
heres one at mooch and a photo of one at huber.
http://youtube.com/watch?v=4pX0ZujgsCE
(photo by frank, who might have video somewhere)