What a great thread! Here's my thoughts on medieval dagger fighting. The two cents are as follows(Aaron, you'll recoginive this stuff as the same stuff I sent you last year):
1)Be aggressive. Move in when ever possible.
2)Control opponents weapon/weapon arm.
3)If possible, dammage the above...remove it's threat.
4)Stab strongly and repedidly.
5)aim for soft areas/unprotected areas.
6)only cut to: the throat, the wrists, the back of the hands, and the forehead/eyes.
7)only cut to set up decisive stabs.

Almost 100% of the time, move to the opponents outside.
9)Throw the opponent/break him whenever possible. Prefferably, throw the opponent onto his stomach
10)Kick, punch, headbutt, claw, ect ALWAYS
11)When possible while training, avoid a "dueling mindest" ie:drawn knife vs drawn knife. Instead, start out in an unarmed state or unarmed vs drawn dagger
12)Most important while training: practice bringing the dagger into play.
As you can see from my thoughts, I differ on some of what has been discussed here and agree with others. Most notably numbers 1, 4, and 11.
Heres why: Number 1. In my way of thinking, it doesn't pay to pussyfoot around. you want to kill the guy. Go in there and get him! You can always follow Silvers advice and get the heck out of range again after you stabbed/bashed the opponent some.
Number 4. Not only does it increase your chances of making contact, but it also puts one into an incredibly aggressive mindset. Does this mean to not make commited stabs? Heck no! It means to stab them strongly as many times as you can.
Number 11. This is a toufgh one because you want to practice the given grapples/breaks/thrusts as they are depicted. But, the danger lies in getting that " dualing mentality." This is where the quick fast snip snips circle circle snip ect happens. Sure some (but far from all period manuals AFAIK) have dagger fighting presented in a juducial dualing setting...but I believe that the vast majority are just like the longsword stuff. That is: designed to use on the field of battle and in self defence.
This last part above really ties in with Jeanries very true observations that we can't let ourselves see this fight through modern eyes. Yes, 99.9% of it translates over to our times...but not the other way around.
Where the manual stuff comes alive isn't in the dueling context, but as many of us have seen, in the sparring/as real as we can get context. In our longsword sparring are we there to snip away at them or to "kill them" as quickly as we can? Why should we view the dagger fight as different?
Jared L. Cass, ARMA Associate, Wisconsin