The first tip for staying motivated as part of your PhD is to write your abstract early. You abstract will act as your personal manifesto to your PhD of the overall goal.
Next, you want to create a vision board for your PhD - this will allow you to visualise everything you hope to achieve in your PhD.
Your statement of purpose is essentially a mission statement for your PhD. Most applications will require you to write one as part of the application - but even if you do not need to do this, I urge you to do so as it will be a powerful motivational tool.
Not sure where the purpose lies in your PhD? Try the Japanese process of ikiagi - finding the purpose of your life, specifically finding the purpose in your PhD
Ikigai - finding purpose in your PhD
Another way to motivate yourself is to continuously update your Resumé or CV to include all of the wonderful things you have achieved as you progress through your PhD.
Similarly your overall research plan can be a great way to do this. This is usually something you will want to develop in first year, and you would need to show this for any early stage assessment.
Coming Soon: PhD Research Plan
Another key area of motivation is goal-setting. The above examples are great ways to get started with long-term goals and creating a vision for your life that you then want to start making small steps towards through short-term goal setting. This section covers that.