For me it comes down to the amount of focus i have while i'm playing. If i'm totally emerged in the game, calling con's out on vox, keeping my SA up, and watching not just the con i'm fighting, but the 3 above and behind him, i usually have a good session.
If i'm even semi distracted, by the phone, two-way on nextel, dog jumping around, friends creating drama for no reason, i find that i'll lose most fights. I'll get suckered into a postion that puts me at the mercy of the enemy, not the other way around. At that time i just log, until i'm in a better mood.