I got this tip from a radiator shop years ago, and it's worked well for me.

Disconnect both radiator hoses from the radiator.  Take out the thermostat and put the thermostat cover and hose back on.  Put a water hose in the top radiator hose about a foot or so, and also put in an air nozzle from your air compressor.  Wrap with a rag to keep most of the water from running out that end.  Turn the water and air on.  You'll be surprized at how much the air pressure makes a difference in backflushing the engine.  

I like to catch the water coming out of the block in a clean tub to see the progress on cleaning I'm making.  

Hope this helps.

RonnieS