I ask to be booked on a later connecting flight than the one offered in the airlines timetable as I prefer a planned longer stop over to missing my connecting flight. Some agents are reluctant to book the flights for me claiming the published connecting times are possible.
On a recent trip LGW-IAH-LAS with Continental (see the thread about them in this forum) we arrived at the gate area in time to see the onward flight they wanted to book us on push back. Our inbound flight had landed on time but due to a ground handling cock up and having our shoes disinfected by the Dept. of Agriculture in customs.
Overall I would have made about 75% of connections on trips to the US had I taken the connection offered in the timetable. My profile explains why I don't mind hanging arround in airports.