Easy one;
Don't be that type of person that gets uppity when they get a bit of criticism from an assessor or in the assessment report. Also, don't be that referee that just looks at the mark but doesn't give a hoot about the actual feedback within.
You want to break the assessment reports down so you can improve. Identify the negatives and figure out the solutions to that. The common ones early on are to do with positioning, fitness, failure to identify fouls, failure to issue cautions. Identify if it was a one-off incident, or if it is a pattern. Identify if the problems can be linked to other areas that you have issues with - for example, were you pulled up for a failure to caution someone? Why? Because your positioning was naff? Then solve the positioning and you'll solve the caution issue.
Beyond that, also pay attention to the positives in your assessments. Identify these, and make sure you keep doing them, even if players/managers/other referees tell you not to. For example; In the majority of my assessments I have been given praise for identifying caution-able offences, issuing the cautions and being consistent with the cautions. Players/managers/other refs think I'm far too harsh, I, for whatever reason this season, listened to some of them recently and in my last assessment I got pulled up for not cautioning appropriately.
In short, listen to your assessors, even in the face of everyone else whining about it, if an assessor says you're great at something, stick to how you do it and pay no heed to anyone else unless it's another assessor.
If you can, get in touch with whoever it is in charge of the assessments, they can be great for clarifying in the case that two assessments provide conflicting advice.
Other than that, listen to their debrief, don't be afraid to answer questions honestly - especially if you made a mistake, show that you're willing to listen and willing to change based on their advice. If they have time, don't be afraid to ask them questions to clarify things, or to identify solutions to things that may be bothering you. Maybe you feel you did badly on something in the match, but the assessor didn't bring it up? Ask him, he may say it's not even worth thinking about, he might give you further advice, but it shows a willingness to learn, which is always good.