Most recently, I've had some luck shooting from the access roads on the south side of the airport while they were doing eastward/8L departures. Oddly even though it was only in Dec. 2012 I'm having a hard time pinning down my location then in Google Maps, but I believe it was somewhere along Archibald Avenue. Note that you may need a pretty long focal length for this.
However, if ONT
is anything like the rest of the SoCal airports (don't visit it often enough to say,) oftentimes operations will be westward, i.e. runways 26L and 26R. In which case you'd probably be better served by parking in one of the warehouse type facilities near Haven Avenue which runs along the airport's eastern perimeter, and heading out from there.
To be quite honest though, I'm hoping someone with more knowledge can add to this as I don't spot there enough or know anyone who does; it's not a very busy airport.