Here's a tip that I have been using to carry my laptop on my commute back and forth to work.

I have a laptop bag that I carry on a rear rack. For quite some time I used a plain rubber bungie cord to strap it in place.. I found this to be very inconvenient on several occasions; when coming to a hard brake, hit a nasty bump or turn too sharply, the laptop bag would flip over the side of the rack and hang on the side.. it never hit the ground but would throw me completely off balance.

After some consideration, I got a cargo strap like the one below and trimmed the excess length down to a manageable size. Now I can put the laptop on the rack, strap it down and tighten it enough to prevent any movement when braking, hard turns or nasty bumps.