Hang from your roof or another hang-over ledge. Start doing hanging exercises. Your grip grows fairly marginally in a short period of time. You'll also notice the time it takes your grip to fatigue grows... Your forearms will also get pretty ripped/large.
This is a fairly common question on some of the parkour forums, and most everybody gives the same answer "hang from a ledge...". I didn't have a pull-up bar, so the closest thing was an I beam at the school down the street from my house. I started doing pull-ups and ab exercises from there, and noticed my grip when biking and other things increased almost exponentially since I started doing those exercises from a beam and bar...