Pack Snacks
The day can get awfully long if snacks aren't provided for adjudicators. Fruit, nuts, and a spare bottle of water are easy to pack and might get you through those last few performances.
Light Source
Having your own flashlight or book light is a great idea. You might get to your space and have a light that's too bright, not bright enough, etc... Having an alternative is a good idea.
Laptop for Notes
There is a growing trend to take notes via a laptop instead of longhand. You might consider setting your software to web format and choosing a black background with white font so your laptop doesn't glow too much in your face. Many adjudicators find it easier to critique from a hard copy so they give the Contest Manager a thumb drive with their notes attached and ask them to print a hard copy prior to the critique.
Adjudicator Tips, Trends and Techniques
Learn some of the ins an outs of surviving the contest play season from some seasoned adjudicators.