You can buy a chest piece that links the two staps together, perfect for houdini's I've seen them at Babies r Us
We have one of these and it works a treat. DD was unbelievable for getting out of her straps. The worst incident we had was on a way to a BBQ one night and I caught a glimpse of her in the rear vision mirror (before I swiftly pulled over!) standing in her car seat waving to the cars out the back window Needless to say, it scared the hell out of us!!