Understanding the mix of karma I'm still struggling with the concept of good karma, bad karma, and both good and bad karma.  If all actions are one of the above- and, as I understand it, mostly the latter, it seems as if we will always be generating bad karma.  Is following the 8-fold path going to insure only creating good karma or will we still be generating both as we struggle on the path?
I think its helpful to bear in mind the different between Karma and it's fruit (or vipāka). An action may have a karmic effect if it has ethical or volitional weight but it's fruit doesn't occur immediately. It might occur years later, in another life or it might fizzle out before it ever occurs. 

The relevance of this is that a person might generate a whole lot of 'bad' Karma but this karma might only bear fruit well after the person has undergone a tremendous amount of spiritual progress. So 'bad' Karma storing up to bear fruit won't stop them on the spiritual path. They might however have to deal with the effects of that bad action when they are an entirely different sort of person.

A good example of this (from the Pali Canon so this has a Therevadan focus now) is the story of Angulimala. He was a notorious bandit and murderer who encountered the Buddha and then became the Buddhas disciple. His 'bad' Karma didn't stop him ultimately becoming a ahrant. However he still had to deal with the fruits of his bad conduct well after his conversion. At one point his was seriously attacked by resentful villagers while on his alms round. Even though he was no longer a murderer he still had to deal with the consequences of his 'bad' Karma.

So just to summarise, your previous bad Karma won't necessarily stop your progress on the spiritual path. But it's likely that you will at some point have to deal with the fruits no matter how spiritually adept you may become.