Suckback porosity in the casting can be eliminated by:
Flaring of the sprue former may act in much the same way as a reservoir, facilitating the entry of the fluid alloy into the pattern area. 
If possible, the sprue former should be attached to the portions of the pattern with the largest cross-sectional areas. 
It is best for the molten alloy to flow from a thick section to surrounding thin areas (e.g., the margins) rather than the reverse. 
This design minimizes the risk of turbulence. Also, the orientation of the sprue former should minimize the risk for metal flow onto flat areas of the investment or onto small areas such as line angles.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 214
Increasing the mold metal temperature difference
Increasing the flow of molten metal
Flaring the point of the sprue attachment
None of the above