One of more successful tactics in Kerbal Space Program in order to get big missions with a lot of delta-V going is to launch the big interplanetary craft (with landers, rovers, service modules and everything needed on the mission) on top of a relatively weak disposable first stage(s). The craft reaches the orbit almost completely depleting its fuel supply, and then consecutive launches deliver fuel, refueling the craft in orbit.
Of course real life is not KSP, but on top of prohibitive cost of such a mission in general (even if launched "traditionally" it would be vastly more expensive) what are the factors that would make this approach impractical?
(which I assume it is, since smarter minds than us decided not to go with it.)