My boss keeps extending the date of my annual assessment for a possible raise. How can I expedite the process and what preparation do I need?

The best approach for this kind of scenario is talk to your Boss and ask him the reason for the extension of your assessment review.  In that case you can hear and come to know the nature of the problem and express you insight with the issue on hand.  never assume unless you have the facts on hand

Thank you for your invite. I cannot guess or assume that it is because of organizational or personal reasons as such things dont happen in our organization, therefore I am not able to suggest but I do agree with Mr. Erixckhzon De Leon suggestion and I endorse his answer. Good Luck.

Be straight forward and request for a private meeting and insist on getting an answer behind the delay. He might be not the one delaying the assessment, the management might be postponing this process for financial reasons.

If the meeting ends and you didn't get an answer, try to remind him after a while.

In both cases, keep your performance at its best.

Provide your boss with an annual report , Mentioning all of your achievements even the mistakes you've done and learned from it and reporting all of the data you have with an acuurate clear statistics to prove that you are working very hard for this company and you want to achieve more and money will only help you as a motivation for your hard work.

You need to focus on your perfomance. Your KRA should be strongly achieved. You should develope more your Skills.

Just you need to raise your performance and achievements 

Step 1: Arrange a meeting (don't say what for, instead say it's private)Arrange to have a discussion with your boss.  Request and schedule some time in his diary.  If he has an Assistant who asks what's it about, insist that it's a private matter and stress that it's very important.  When the time has been agreed begin your preparation for the meeting.Step 2:  The preparationMake a list of all the additional responsibilities you've taken on.  Identify what the market rate is for someone who's doing the full role that you're doing (look this up online - search for salary surveys).  Collect facts about the business' growth.  List examples of how what you've done has contributed to that growth.  The more data you have the better.  Arrange the data into some meaningful information that supports your case.  Type up the information into a report format with charts, graphs and forecasts.  Forecasts explain how you expect your continued efforts would contribute to growth.  NB.  Projections are rarely linear.  Research what I mean by this so that your report really reflects the exponential nature and possible long term impact of your role.And it goes without saying but I'd say it anyway.  Any preparation for this meeting should be done on your own time.  At home, not at work.Step 3:  Present your case at the meetingBefore you whip out your report, say something like this: 'I appreciate that you've been very busy lately and I don't want to take up too much of your time.  There is however this incredibly urgent, yet personal matter that needs your input.  I've been with <<state the company's name>> for <<state tenure>> and I enjoy working here.  I'm being challenged and I like that.  My concern is that I'm not being compensated in line with my current duties.  And I'm unsure that there is a regular process to assess my performance and remedy any pay gaps.  This concerns me and I'd like to discuss.  I've done some research and put together this report which .... <<this is when you take out your report and explain what impact you've made to the business to date and what you estimate might be the results in the future>>'The number one thing is to be professional.  Present your case, discuss their thoughts on your presentation and then agree a way forward.

Keep your skills improving and showcasing your performance at all the time and keep your seniors updated on what work you are doing by giving regular uodates to them.

Raise your skill sets and work on soft skills specially. Your performance is a key with the proper way to express it infront of your management 

Thanks 

Just you need to raise your performance and achievements