I tend to do both quite often, so I can't say I dwell on one more than the other. I like to take experiences from the past and apply them to the present in the hope that they'll help me make better decisions going into the future. :)
Though when I look to the past it's often in a negative light, often with lines like "I should have done that" or "I shouldn't have done this." However when I look to the future it's more like "I will do this" and "I won't let that happen," so as a result these thoughts are usually much more positive. ^_^