like they said, it's all in the timing of the jump then crouch, OR you can hit crouch, jump, crouch, they both work. find an empty server and just keep trying it. the boxes in mid are harder to jump on and the 2 high boxes in A are even more diffucult, until you get the hang of it. the ones in A you have to be all the way to the left, do the same juming routine to get to the first ledge, then scoot over just a smidget to the right and do the same jumping routine and whala! You are on top of the world!
the jumping ones that i just cant seem to master is the eves in backyard in office. i can sometimes get to the first eve, but usually don't make it to the tree let alone the next eve and the next eve and then finally the last tree. I have made it all the way a couple times but watch some guys do it almost every single time without fail. i guess it's like anything else, practice practice and more practice.