Back pressure porosity can be avoided by:
Backpressure porosity 
This is caused by inadequate venting of the mold. Air is trapped in the mold and is unable to escape. The sprue pattern length should be adjusted so that there is not more than 1/4" thickness of the investment between the bottom of the casting ring and the wax pattern. When the molten metal enters the mold, the air inside is pushed out through the porous investment at the bottom. If the bulk of the investment is too great, the escape of air becomes difficult causing increased pressure in the mold. The gold will then solidify before the mold is completely filled resulting in a porous casting with rounded short margins. 
Avoided by:

Using adequate casting force. 
Use investment of adequate porosity. 
Place pattern not more than 6 to 8 mm away from the end of the ring.
Providing vents in large castings. 

Manapallil 3rd P:370
Using asbestos liners
Placing the sprue at least one quarter inch away from the end of casting ring
Preventing rapid heating of the investment in burnout oven
Using a short and wide sprue