You're right on the lowest gear part. Low/high has to do with gear ratio (front gear/ring tooth count divided by rear cog tooth count).

Shifters pull a derailleur back and forth and basically force the chain off one ring and onto another. Index shifting assumes your cassette/freewheel cogs have a certain distance between them and the shifter is designed with this information to jump between the different cogs in the back. Friction shifting just moves the derailleur without any knowledge of the gears, so it's up to you to determine when you've shifted enough. Front shifters (left side) are usually always friction; I don't think front index shifting was really common for downtube/bar-end shifters (or until brifters came out).

As for shifting, I've found that for modern chainring setups, your shifting strategy seems to work well enough. Ideally, you'd want to shift such that your gear ratio goes smoothly up or down. I find this usually means I shift to the large front ring if I need a big jump in gear ratio, and then shift the small back rings if I need less of a jump. For city riding, I almost never need to worry about shifting between the front rings, but for longer rides, it helps a lot, esp on hills - getting momentum by shifting to high gear during downhill really helps. Other than that, the only other strategy I can think of is less shifting and more just pacing yourself - know your limits, don't push too hard at any one point or you'll wear out early. Sheldon Brown had some good advice too - as a rule of thumb, you should always be pedaling around the same cadence, so adjust your shifting so that you can maintain that cadence. It can differ with different chainring setups, like half-step gearing, but the principles should be the same.

Alright, that's my schtick. Someone who knows more than me educate us all some more