I'm doing this...
http://www.ezbabysaver.com/
My wife and I have lived our own little lives for few years, but now we have a baby on the way. I'm known for being forgetful, so I'm a little worried about not remembering a baby is in the backseat. I'd rather be the dumb guy with a door strap than have a dead child.
A bungee cord should work nicely.