I don't know if this is a repost, I searched the forum but didn't find any thread with it.
Most of you might already know this, some might not, so here goes:
To keep it simple, find a corner of a building, or a place where two walls meet each other, point your character directly into the corner (or whatever you found) and jump into the wall, I'll post a series of screenshots, and perhaps a video later. Don't give up if your first jump didn't work, keep jumping into the corner (or whatever...) and you'll see. It usually takes me anywhere from 1 - 15 jumps.
Hope this is not a repost, and I hope this helps you guys .
Screenshot series:
Sorry if the screenshots are a bit big, I already resized them a bit