Effect versus Affect

Dictionary Definitions:
Effect: Something brought about by a cause or agent; a result.
Affect: 1.To have an influence on or effect a change in: Inflation affects the buying power of the dollar.
2.To act on the emotions of; touch or move.
3.To attack or infect, as a disease: Rheumatic fever can affect the heart.

Quick and Dirty Rule:
Can you replace "effect" with "result" and have the same effect? Might also replace with "cause".

Some rules from yourdictionary.com

1. If you are talking about a result, then use the word "effect."
•Example: What effect did the loss have on the team?

2. It is appropriate to use the word "effect" if one of these words is used immediately before the word: into, on, take, the, any, an, or and.
•Example: The prescribed medication had no effect on the patient's symptoms.
•Example: In analyzing a situation, it is important to take the concepts of cause and effect into consideration.

3. If you want to describe something that was caused or brought about, the right word to use is effect.
•Example: The new manager effected some positive changes in the office. (This means that the new manager caused some positive changes to take place in the office.)

4. Affect can be used as a noun to describe facial expression.
•Example: The young man with schizophrenia had a flat affect.
•Example: The woman took the news of her husband's sudden death with little affect.

5. Affect can also be used as a verb. Use it when trying to describe influencing someone or something rather than causing it.
•Example: How does the crime rate affect hiring levels by local police forces?
•Example: The weather conditions will affect the number of people who come to the county fair this year.

Reference: http://www.yourdictionary.com/grammar-rules/affect-effect-grammar.html