That is a hard question.
Start by unloading and laying it all out. Make a list of everything in separate catagories/groups. eg signalling, emergency devices, water and food etc. See the Kits links etc from the ETS home page for what I mean.
Then revue. What is missing? what can be improved? What do you need more of?
If you cant think of anything pack more cotton balls as tinder and to stop the rattles, or upgrade to a smaller tin.
Actually I dont think I have see anyone here with the problem of having with to much space to fill before <img src="/images/graemlins/smile.gif" alt="" />
Not knowing what else you have in there makes it a bit hard to narrow things down any further.