How can the runway allocation process be optimised?
This question is extremely broad. At airports where there's multiple runways, there are numerous factors influencing which aircraft get which runway. Factors include, noise sensitive areas(so one runway should be favored over others), runway length/operational necessity, and configuration.

When an airport has multiple runways usually the tower will decide which are being advertised as arrival/departure runways. At which point the approach control will sequence to those given all the restrictions. Runway assignment usually occurs upon the aircraft entering the approach control's airspace. Once assigned to a runway, it's up to the controller to decide on the sequence, factors include airspeed, distance, and somewhat weight category. If you have multiple heavies coming in, it's often better to group them, than mix small-heavy, BUT you need to have the volume to be able to setup that kind of mix, to minimize space.