The xp system I use in my campain needs some book keeping, but I find it works if you want a slow progess over many games.
Each time a mini causes a wound on an ennemy (either taking down a mini or causing a wound on a construction), the mini gains an xp. When the mini has as many xp as it's current cp cost (not counting equipment), it gains a 1 cp ability (ex: a normal mini manages to kill 4 opponents in a battle, at the end of the battle it would "level up", and could gain the "Thick Skin" advantage, gaining +1 armor).
Thats the basic. You should customize it for your own play style.
For exemple, if you have special objectives, the mini completing the objective could get a point.
If you use many squads (and don't shift mini from one squad to another), you could total the xp for the whole squad, and give the level up advantage to the entire squad (although, you might want to halve the level up cost, and add that the squad benefits only if at least half the original members survives).
Oh, and the list of advantage is in the 2001 rules, but with some additions related to my own house rules.