Power: New buildings are powered off. Unpowered buildings do nothing for you. Powered buildings do their job automatically. You will rarely have enough power to lit everything up. One of the most important decisions you can make is how many buildings (and which ones) to power up. Click on any building to bring up its power panel.
Maintenance: Each building needs periodic repairs to operate effectively and resist demolition. Maintenance is done automatically by your workers, and consumes several types of resources. The more buildings you have, the higher this consumption will grow. Be careful not to leave your buildings in disrepair, or your economy will stall.
Mix: Having the right set of letters at each stage of the game will do wonders for you. Too many unneeded letters will drag you down. Generally, you should begin by building vital production and power letters and beef up services, before you invest in defense. The location of each letter relative to others is pretty important too.