Tips for Gathering Resources

You primarily need resources to train and upgrade  your survivors. Resources include silver, other survivors, and training material.

The quick tips are:
 * Use your energy to "farm" for resources in the Hospital or Military Outpost map locations. Those two locations have lots of zombies that only take 1 energy to kill and the kills have high chance of dropping survivors or training material.
 * Get your daily free survivor from the Crates section of shopping.
 * If you find that it takes two hits to kill zombies when doing missions, take a break from doing missions and farm for resources so you can train and upgrade your survivors until they are strong enough to do one-hit kills.

Buildings = Silver
When you first start playing, focus on buying and upgrading buildings so that you can have a decent income of silver. Slow down on the construction once buildings start costing more than 2,000 silver, although you should still consider buying a new building periodically.

Here is a google doc with a breakdown of costs and income for all the buildings.

Missions
Missions are a way to level yourself (as opposed to your survivors) and to get crates that contain survivors. It is up to you whether you want to focus on doing missions, but once your missions go to map locations beyond the Hospital and Miliary Outpost, you will find that you get more resources by farming than by doing the missions.

Farming = Survivors and Training Material
You need lots of training material and sacrificial survivors to train and upgrade  your main survivors. The fastest way to get these are to go hunting in the Hospital or the Miliarty Outpost maps because the chance of a kill dropping either a survivor or training material is higher in these two maps. Hunt only the zombies that take a single point of energy to kill because you want to maximize the number of zombies killed with your 100 points of energy.