Zombie Farm [iPhone]
Plow empty land to gain 1 experience point but lose 10 coins. Sell the plowed land to get the 10 coins back while keeping the experience. This can be repeated as many times as desired, but is less effective in later levels because of the low amount of experience gained.
After planting something that takes a long time to grow (for example, two hours for a tomato or two days for a potato), exit the game. Go to the system settings and advance the time for the amount needed for the plant to grow. Note: This may kill any fast crops if done too long.