Major cause of paging discard is due to physical channel PCH congestion.
The capacity of PCH channel is indicated by: how many paging messages it can by transmitted per second/min/hour?
So, to avoid paging discard we can do:
1, Reduce number of paging messages sent over air interface: split TAC, reduce number of cells, or active feature smart paging at core network ( first paging on the last site/cell UE camped on, then if failed paging whole TAC)
2, Increase PCH capacity: by change some parameters as above.