Think about the future. The more complete and clear your mental picture is of success, the easier it will become to stay focused and keep going.
Imagine overcoming your current difficulties. It may not be obvious how to do it, but by focusing on the pleasures of success, you are more likely to feel motivated to keep trying.
Think of someone you admire. When the going gets tough, what would they do? What would they advise?
Look at past experiences. Remember another occasion when you faced a challenge that seemed insurmountable. How did you cope then? The situation usually looks a lot worse when you're stuck in the middle of it. Recalling how you survived last time helps build your resilience this time around.
Write down your achievements. At the end of each week (or day), jot down everything, however trivial it may seem. This will take longer than you imagine, usually a lot longer. It will help you feel more positive about the future.
Focus your energy on things you can affect. Once you have decided on a course of action, stop thinking about it and move to the next challenge.
Use your social support network. The prize is for overcoming challenges, not for doing so alone. Friends and colleagues are usually willing to offer new insights or emotional back-up.
Look after yourself. Your health is often the first thing to suffer and and that can devastate your resilience.
Note that not everyone had it easy. Michael Dell and Pret A Manger founder Julian Metcalfe didn't. To make it to the top, you need to keep going in the tough times.
Believe that the only way is up. Remind yourself that the worst scenario is that you learn a lot.