For anyone who is interested in learning past life astrology, I HIGHLY recommend the book by Steven Forrest: Yesterday's Sky. It is a wonderful book on how to use North and South node to track down your past lives and see how it influences your life today. It is a fascinating read and I got so much out of it, it is absolutely wonderful.
I agree that Saturn influence can signify many previous lives and show that you are a "mature soul". However I would not say that it signifies necessarily bad karma. It just signifies that you've done all of this many many times or accumulated a lot of understanding and maturity in your previous lifetimes. Challenging aspects from Saturn can tell of karma that has to do with deprivation, extreme weather conditions, challenging periods of time and authority figures that were cold and unemphatic.
What I have learned the most is that if you want to understand your past lives better, you should look at the ruler of your SOUTH node and it's aspects on your chart and "map" how it is placed on your chart. This can tell a lot of the themes that you had in your past life. Also, everything you need to know about your past lives is clearly seen on your chart. You don't need to find any fancy stars to guess work. If you had issues with your femininity in your past life and if you came here this time to do it right, you will see it clearly on your chart, there is no doubt about it. Venus in hard aspect to Pluto is a clear sign that in this or in the past lives person has had, there has been issues with control and sexuality in relationships and you have the same pattern in this lifetime and you can now take another look at it.
So the life you are living now, is very much a continuation from your previous lifes and your current chart will show that. Sometimes going deep into North-South node symbology, you can find a lot of detailed information of your past life themes and that might open up your current chart in a whole new way.