You could try to gamify the process
I didn't try this, it's just an idea but long questionaries are usualy arduous work with delayed gratification. You could try to give back small value every couple qustions.
In psycho-tests, you could give a partial response after each 10 questions:
- "It seems that you have attentive personality. How would you react in following situations, considering that [...]"
In career-profile builder you could use small chunks of feedback like:
- "You seem to have a lot of experience in technology. When facing interpersonal problem, would you rather [...]"
- "You seem to value trust in your working enviroment. How important are following skills, if [...]"
In general people like to learn about themselves or get validation for things they believe. Good insight is always appreciated.