Just as the new Familiar Templates are used for designing the basic familiar, so this system is used to improve the familiar as the master gains in level. Each level the master gains in his spellcasting class that allows for familiar improvement gives the master a choice of 1 of the many enhancements for his familiar below. Some of these will have certain prerequisites, so not all are immediately open to him.
Each level in wizard or sorcerer (or any other class allowing familiar advancement) the master gains, including level 1, gives him 1 pick from the following enhancements (can choose from either of the 2 lists). Those abilities of the same name as the normal familiar enhancements work the same way. You can mimic the Standard Familiar enhancements with this system, gaining them all by level 19 (so you end up with 1 extra bonus by level 20). Some of these have requirements that will not allow the master to choose them until his meets the requirement:
Standard Enhancements (as in the PHB)
Familiar INT/CHA: +1 INT & CHA for the familiar. The familiar's base INT/CHA cannot exceed the master's primary spellcasting ability score.
Familiar Toughness: familiar gains a natural armor bonus of +5
Familiar Scrying: requires Familiar INT 13+, master gains the Scry on Familiar ability
Improved Familiar Bond: requires Familiar Bond and Familiar Scrying, master can see and hear what his familiar sees and hears as long as the familiar is within 1 mile. The master must concentrate to get this information and it uses up one of his Scry on familiar abilities, lasting the same duration as his Scry on Familiar lasts.
Improved Familiar Resistance: requires Familiar Resistance, familiar gains +6 to his SR.
Improved Familiar Speech: requires Familiar Speech and Familiar INT 12+, the familiar can speak 1 or more languages, speaking 1 language for every point of his Familiar INT bonus. This is in addition to his ability to Speak with Master and Animals of Its Type.
Extra Scrying: requires Familiar Scrying, the master can scry on his familiar 1 additional time per day. This choice can be take multiple times, its effects stack.
Improved Familiar Scrying: requires Familiar Scrying, Extra Scrying and Familiar INT 15+, the master's ability to scry on his familiar is enhanced and now acts like a Greater Scry.
Improved Familiar Arcana: requires Familiar Arcana and Familiar INT 10+, the ability to Share Spells is no longer restricted to the familiar remaining within 5' of the master, the familiar can now go as far away as 1 mile from his master and still share a spell with a lasting duration. The familiar must still be within 5' when the shared spell is cast. The master can also transfer a Touch spell to his familiar, the familiar must be within 5' at the time of the transference, but the master touches his familiar and the touch spell now resides upon his familiar. His familiar can deliver it normally, but must deliver it before trying to deliver another or it is lost. This allows the master to cast another spell without losing the touch spell, as it is no longer on him, but on his familiar.
Familiar Feat: the familiar can choose any feat it can qualify for, like Weapon Focus (natural attack), Dodge, Mobility, Expertise (if INT 13+), etc.