Be sensible when awarding milestones to players, particularly those who may access powerful stunts. Force players to be economical, particularly when looking to expand their magical capabilities.
In the case of significant milestones that would normally permit a player to increase a profession’s rating by a point you could also allow that player to improve an existing magical stunt, either adding a new signature spell available to that archetype or improving a spell. So, for example a spell that would normally only be usable once per scenario becomes available once per session and so on. Be careful when allowing players to increase shifts of damage attributed to spells and respect game balance.
Of course a major milestone will grant a player a bonus fate point which can be immediately spent on a new stunt, which might allow that player to complete the roster of signature spells available to them or indeed add a new mage archetype providing they have an aspect that justifies it. Just what amount of gold is necessary to buy these milestones will be determined by the Dungeon Master and campaigns can vary from one group to another. It’s also worth making temporary fate points available which may be bought during play – these points vanish from the refresh pool as soon as they are spent.