If the preceding sentence contains anticipatory phrases or words like as follows, thus, these, etc., use a colon:

The German shepherd was trained to respond to these

commands: 

• Heel
• Attack
• Recover

or 

Our approach has the following drawbacks:

1. It conflicts with our technical strategy.
2. The cost structure leaves no margin for error.
3. The risk factors have not been realistically addressed.

When the sentence is lengthy and the anticipatory word occurs very early in the sentence, use a period:

The following steps may be initiated at any time during

the procedure to ensure the viability of the final outcome.

1. Inspection
2. Analysis
3. Corrective action

In paragraph lists where the list follows a preposition or a verb, do not precede the list with any mark of punctuation:

Our customer will award the contract on the basis of (1) our

perceived strengths, (2) our perceived weaknesses, and

(3) our competitors' perceived strengths and weaknesses. 

or 

We must (a) look sharp at all times, (b) always be courteous

and kind, and (c) never lose our composure. 

In a displayed list, use a colon when the list follows a verb:

The three criteria addressed in the analysis are: 

1. Understanding of the problem
2. Soundness of problem approach
3. Extent of compliance with program requirements

NOTE: Although the above example is quite acceptable, some editors would prefer that the lead-in sentence be rewritten as follows:

The analysis addresses three criteria: 

1. Understanding of the problem
2. Soundness of problem approach
3. Extent of compliance with program requirements