The first one that comes to mind for myself is something like:
For SB Fold: -2 For SB Limp: -1 For SB 2x : 0 For SB 3x : 1 For SB 4+x : 2
Preflop Aggression = (sum of all recorded actions)/(# hands played)
The assigned values for the actions could be tweaked slightly. I definitely think that folding and limping both qualify as negative aggression though. A minraise isn't strong enough to actually induce a fold, so it doesn't really score as aggression. Real raises qualify as actual aggression on the scale.
I like this thinking, but let's expand it a little bit more...
if he limps from small blind and calls your raise: 0
reraise you after he limped: 2
if he limps then folds to your raise: -1 (or maybe -2)