I don't think anyone has mentioned this one yet, but talk to your RO. Tell them you are aiming for a good assesment and ask their advice for what you need to do to get it. You can do this at the start of the reporting period then when the 6 monthly comes around ask if you are still on track for a better assesment.
Also make sure you know what needs to be on your assesment! for example the 2 Ranks up I was told needs to be at least a 'Yes' to be considered. My 1st RO didnt know this, but was happy to change it when we discussed it.
Finally, if you don't agree with your report, tell them! It can be hard, but there are stories from people all around the RAF who got picked up after doing that.