Congratulations, mate - well done.
I've done three years' worth of district assessments with East Devon (obviously the undisputed kings of district cricket in the county...) and am about to start my second year of the same with North Oxfordshire. They can be interesting and revealing days on all sorts of levels, but with experience as a coach/assessor, my first impressions are generally right on target and there are a number of things that I look for beyond the typical skills.
Dress - go in whites, ideally with your club crest on, and definitely clean. Make sure your shirt's tucked in. Don't turn up in a football shirt. Particularly not one with Ginsters on the front.
Arrival - get there early and introduce yourself to the coach/manager. If you have time to kill, do some throwdowns or catching practice: make them appropriate practice, not wild slogging.
Kit - if you don't own your own equipment it's not the end of the world, but if this is the case then let the manager know early and make it as easy as possible for them to arrange a work-around.
Attitude - I can guarantee the assessment will start with a briefing from the manager/assessors. They will explain everything that you need to know and won't be too interested in interruptions, players who aren't listening, or daft questions. If there's something you want to ask, wait until the very end of the speech and the time you are asked for questions, and ask politely.
Banter - District trials often have a bizarre social dynamic of the County players, established District players (is this for U13 next year or U14 next year?), and new boys - it can be very, very quiet and as a coach you notice very early on which boys are the leaders; who will encourage and support others when they need it? Who will give a shout of "great stop" or "good arm" during the painfully quiet middle practice? These are the things I look for to split line decisions later on: who do I want to spend time coaching this winter - who will make the most of it?
Lastly, do the whole thing with a smile on your face, go out to enjoy it and say "thank you" at the end. There are an awful lot of kids to see in the course of the weekend/week for assessors (I must have seen 200+ last year in Oxfordshire) and it does often come down to marginal calls. If you have created a (positive) reason for the staff to remember you, then you will be well set.
