How were pilots selected and trained before the introduction of simulators?
Almost every pilot learns to fly in an actual aircraft, this is one of the reasons all trainers have dual controls.  



My first day out, ever, was in an actual plane, behind real controls, in the sky. No sim-time prior to that. The instructor talked me though takeoff and maneuvers (keeping an eye on everything and operating the rudder). All he did was radio coms, navigation and the approach/landing.   

As for whats common: 

The vast majority of American WWII pilots are said to have learned to fly in the piper J-3 Cub

The Boeing PT-17 Steraman and AT-11 Kansan were also popular. 

In GA where everyone else gets their start the Piper Cherokee and Cessna 150 or 172 rule supreme. 

Of course these planes are slow, benign and fairly safe by aviation standards. But what if you have some super fast, one of a kind, high flying, awesome supersonic jet gadget. You surely cant afford to ruin such a plane but still need to train pilots. Well in that case you just build yourself a dual cockpit custom trainer.... 

  

Even the space shuttle had a real flying trainer designed to mimic its approach characteristics. 

All that being said simulators can be a big help if used properly.