Determine what you want to plan or prepare for. The future is a big place with many eventualities, but chances are that you want to address a specific situation,problem, or opportunity. Define this end to the best of your abilities. 

Use your intuition. Not all decisions are rational or carefully analyzed, and intuitive guesses can often be quite powerful. What feels right? What do you think will happen? When you use your intuition, you draw upon your experience and knowledge in a different way than when you make a rational analysis. 

Consider what you already know. Prior knowledge comes from many places. Have you tried something similar before? Do you know how somebody is likely to react? Have you seen something done or could you read about others' experiences with a situation? Could you ask others? Can you try something out or gather data that might suggest what could happen? 

Detect your own bias. People tend to bias their guesses and actions in certain predictable ways. For example, recent events may play a larger role in influencing decisions than they warrant; or, you may be more likely to believe something just because everyone around you believes it. If you think this sort of thing is happening, start looking closely at hard evidence (like facts and numbers) and question your own assumptions. Consult the list of cognitive biases  for common presumptions and biases and see if any apply to you. 

Invent hypothetical situations related to your objective. Ask yourself "what if" for various possibilities and imagine possible outcomes, possible courses of events that could result. Especially, think about possible consequences of different courses of action. 

Consider the worst-case scenario. What is the worst thing that could possibly happen? Evaluate the possible risks. 

Consider the best-case scenario. What is the best thing that could possibly happen? Evaluate the possible rewards. 

Think of possible actions to take. If you are trying to think ahead, it is probably because you want to decide how to respond to some situation or need, so think of possible responses. 

Evaluate those actions. Based on your experience and knowledge about how such events usually turn out, choose or narrow down which action to take. 

Try it. Act according to your forecasts and your plans. Then, let life take its course. See what happens. By taking note of the outcome, you will have more experience and knowledge to draw upon the next time you must make a decision such as this one. 

Adjust. As you see what really does transpire, adjust your actions or responses as best as you can. You may not have the opportunity to change course after you begin, but you do have the benefit of acquiring new information or results. Use them to decide how to modify your actions in the present and in the future.