internal plaster cracks

Plaster above windows and doors is more susceptible to cracking than walls because of the smaller areas involved and thus susceptibility to stresses. Not much to be done about this. You haven't described your window type but if they are wood frame where the you open/close the internal (and lower) window vertically, ensure that the friction of the moving window is minimized (your can rub bar soap on the frame when the window is up).

Our home was built in 1955 over clay, and we always get cracks near the doors and windows after we have some rain. Not many; only on particular doors and windows. The ground must shift up and down with the rain, and the shifting of the foundation must be uneven. I get cracks on the corner of walls too.
I've become accustomed to doing maintenance on it every year during the rainy season. It's not a big deal to me.
Some decades ago, our neighbor across the street had really bad problems where the ceiling was slanting and cracked so badly, she had her whole house lifted by a crane and redid her foundation.

Here's an elastic dry wall repair compound that sounds like a good option.
http://www.homedepot.com/p/Goof-Off-4-oz-Goodbye-Cracks-Elastic-Crack-Cover-Spray-FG695/202022016