The most common complaints I've heard regarding Archmage are that the AP cost is too steep and that the cost to acquire the higher level SLAs is too prohibitive to be a worthwhile expenditure. Part of the problem, imo, is the see-saw jumping of gaining mana for the base PrE and then spending those SPs for the abilities.
The information is a lot of repetition, so I will skip much of the tediousness and try to focus on the gist of the changes.
Here is how I recommend the PrE be changed:
Archmage I: <Spell School> - 4AP
Prerequisites: Level 6 Wizard, Wizard Intelligence I, Wizard Concentration II, Wizard Energy of the Scholar II, Mental Toughness, and any Spell Focus
Benefit: You are an advanced practitioner of <spell school>, allowing you to bend spells in unexpected ways. You gain a +1 to the DC of spells from <spell school> school.
SLA 1st Level - 1AP
Prerequesities: Archmage I: <Appropriate Spell School> or Archmage III: <appropriate spell school>
SLAs cost 1sp to use, reduce max SP by 30
Archmage II - 1 AP
Prerequisites: Level 9 Wizard, and of Wizard Archmage I <Spell School>, Wizard Intelligence II
Benefit: You gain the ability to focus your will into raw energy, launching a bolt of force that strikes one opponent for 1d6 damage per caster level.
SLA 2nd Level - 1AP
Prerequisites: Level 9 Wizard, Wizard Archmage II, SLA 1st level
SLAs cost 3sp to use, reduce max SP by 35
Archmage III <Spell School> -1AP
Prerequisites: Level 12 Wizard, Wizard Archmage II, Wizard Concentration III, Wizard Energy of the Scholar III, Spell Focus: <any school except school chosen for Archmage I>
Benefit: You are an advanced practitioner of <spell school>, allowing you to bend spells in unexpected ways. You gain a +1 to the DC of spells from <spell school> school. You cannot choose the same school as previously chosen for Archmage I.
Archmage III - 1AP
Prerequisites: Level 12 Wizard, Wizard Archmage II, Wizard Concentration III, Wizard Energy of the Scholar III
Benefit: You have chosen to forgo a secondary school mastery and focus instead on mnuemonic exercises, increasing your max SP by 100.
SLA 3rd Level - 1AP
Prerequisites: Level 12 Wizard, Archmage III, Archmage III <Spell School>, SLA 2nd level
SLAs cost 6 SP to use, reduce max SP by 40
Archmage IV - 1AP
Prerequisites: Level 15 Wizard, Wizard Archmage III, Wizard Intelligence III,
Benefit: You gain the ability to focus your will into raw energy, launching a bolt of force that explodes on impact dealing 1d6 damage per caster level to all enemies near your target.
SLA 4th Level - 1AP
Prerequisites: Level 15 Wizard, Archmage VI, SLA 3rd level, GSF <School>
SLAs cost 10 SP to use, reduce max SP by 45
Archmage V <Spell school>: - 1AP
Prerequisites: Level 18 Wizard, Wizard Archmage IV, Wizard Concentration IV, Wizard Energy of the Scholar IV, GSF <appropriate school>
Benefit: You have achieved mastery with <School> magic, granting you another +1 bonus to your spell DCs of that school.
SLA 5th Level - 1AP
Prerequisites: Level 18 Wizard, Archmage V <Spell School>, SLA 4th level
SLAs cost 15 SP to use, reduce max SP by 50