Noel: That depends on how much luggage you have.
Walking -- If you travel light, can each carry your own bags, and don't mind walking, this would be the simplest way.
Taking a train -- You could take the Metro from Gare d'Austerlitz to Châtalet, walk to Châtalet Les Halles RER station, and catch the RER A from there. (It's one station before Gare de Lyon.) To do so, take Metro line 10, transfer to Line 7 at the Jussieu station, get off the Metro at Châtalet. Then walk along the connecting corridor from Châtalet to the RER station. IMO, this is NOT a good option -- these particular stations are huge and a logistical nightmare (they serve 5 metro lines and 3 RER lines). The walk alone to the RER station would take longer than if you crossed the bridge in the first place.
Taxi -- If you have more luggage than you can comfortably handle, a taxi is a good option.
Woody