Successful psychology dissertation depends on two main factors:

• Student’s desire to write a psychology dissertation;
• Topic of a psychology dissertation.

I think that if you are still going to write a psychology dissertation, then our attention will be focused on choosing a topic for your psychology dissertation.
 
When choosing a topic for your psychology dissertation, you should take into account the following points:

• Relevance of the topic of your psychology dissertation, that is - it should be significant in terms of the investigation;
• Degree of interest to the topic of your psychology dissertation, that is - this topic should be catchy and captivating for you and its future readers;
• Psychology dissertation topic’s application, that is - it should be fruitful, so that you did not waste your time on doing some useless things.

As you can see, you should consider a lot of variants when choosing a topic for your psychology dissertation. If you make some survey during your investigation, it will be twice beneficial for your psychology dissertation.
 
Psychology presupposes some research on the mentality of people. You are welcome to carry out some experiment to find out how people behave in this or that situation. You can compare and contrast people of different age, race, nationality, sex, etc. to make your experiment as interesting as possible.
 
In conclusion, it is worth stressing that you should not forget to follow the standards for the dissertation writing set by your department. These standards can differ, so, you should clarify everything with your tutor. AND! Do not forget to proofread and edit your work. It is not only important, it is even obligatory! You do not want silly mistakes spoil your work!