How to earn silver in Avengers Alliance?

I have found that I can earn silver from doing flight deck missions, normal story mode missions, and visiting friend's sections of town.

The problem is that I am not earning enough at a decent rate to keep my heroes training and using time efficiently.

Are there other methods of earning silver so that I can continuously train/research?

Solution 1:

After playing for over a month, I've found there to be no other ways to earn silver in the game (aside from paying for it).

The most effective method however is to run constant 3 minute flight deck missions, regardless of level. They bring in the highest amount of return for each passing minute.

Source/First hand data