With the convention just round the corner, I thought that a timely reminder of all the paperwork would be in order. This can all be found here.
From my personal experience last time round, here would be a few hints.
-Spend some time creating a folder of information on the prototype you modeled. This apparently makes up a chunk of marks under the guise of 'research'. There's extra marks for creating your own plan (I think) which sort of makes sense but more applies to bits other than the trains (which are more exhaustively covered).
-Indicate clearly which bits are hand made etc. Many moons ago I entered a German armoured 'train' in a competition, and it was judged as a kit as I had recycled a Lima power bogie and an old tank turret, despite the fact I has made the bulk of the body and hand detailed it.
-Make sure that the judges know how solidly built the model is. Apparently the judges were afraid to handle my J sheep wagon.