Some memories are wonderful to think about, and can be a great tool to use too.
If you feel down or demotivated think about a time when you were happiest. Anytime in your life, a memory that is strong enough so that when you start noticing what you saw, heard and felt in that memory the wonderful feelings start trickling in...and with every breath they become stronger and stronger as you notice what you were saying to yourself at that point in time too.
Notice what is different now...what has changed in terms of your physiology, thoughts or feelings just by reliving that memory?
What did you say to yourself when you were totally happy?
Who/what was present?
Was anything missing from that memory that is usually present in your daily life?
Anchoring is often used in NLP, creating a link between a stimulus (visual, auditory etc) and a feeling from a memory. Therefore whenever you use that trigger again, the feelings automatically crop up too.
Anchors can happen naturally - have you ever heard a song and then recall a memory along with feelings related to that song? This is an example of an auditory anchor.
By reliving a particular memory whilst clicking your fingers, listening to a great song or looking at an inspiring image you can create a powerful anchor to use anytime you wish to recall those feelings. The more times you do this the stronger it becomes.
Try it and notice how great it can feel!