Bicycle Help?

As my normal sources of cycling info are rather busy, anybody out there who can help me with the following?

Until recently, I hadn't ridden a bike since I was a teen, and then it was usually at a rather leisurely pace. But now I'm back on a bike, and using it for a more serious workout. However, I'm finding that things like technique seem to matter a lot more than they did as a kid.

So, I'm begining to focus on making a very smooth pedaling motion that uses both legs equally (I think I rely on my left more), and I'm trying to figure out the right posture. To this end I've been doing some reading, but it's all very expert-oriented, and references only roadbikes. It's hard for me to understand, and I'm not sure how much of it applies to my more offroad-style bike. I'm also getting numbness on the 3rd and 4th fingers of my left hand after only about 15 minutes on the bike (no matter which bike I ride), and I can't seem to shift without slipping gears.

Any advice?