Basically you use present perfect/past perfect when you're actually interested in the fact and its consequences, rather than when it happened.
Eg.:
I had eaten dinner before I called a friend.
We don't care when exactly you ate your dinner, what's important in this sentence is that you had your dinner before another event (here: your calling a friend). The fact that you had eaten your dinner has presumably some impact on the time you called your friend (eg. you won't call him to go to the McDonald's).
Another example: I have lost my keys. We don't really care when I lost them, but rather that it's annoying now. However, if I add 'yesterday' for instance, the past simple is mandatory: 'I lost my keys yesterday'.
Back to the consecutive events examples (first...then), the second part helps locating the situation in time, thanks to the past simple. We now that the described situation took place when you called a friend of yours.
First I had eaten dinner, then I have called a friend. <--incorrect? why?
First I had eaten dinner, then I had called a friend. <--incorrect? why?
The problem is that we expect a past reference so that we know when you had eaten dinner. Unfortunately, the second part "I've called a friend (of mine)" doesn't help. So you end up with two events that are not bound to any specific moment -- wrong.
Again:
I have lost my keys
=> important now
I had lost my cellphone when the accident came about.
=> important when the accident came about.
FRC