I just got back from judging elementary level science fair (this is like kids in 6th grade or something). Here are some quick notes in no particular order.
- Please, please do not have a 20 item list of supplies that you used on your board including things like thumb tacks, tape, paper and poster board. And then please do not read this list aloud during your presentation. I know you are nervous, I feel for you. Maybe you are trying to fill up time - maybe you think a long presentation is a good one. However, don't do it. Stick to the important stuff. If you need to fill up space on your board, I would much rather see a picture of your experiment.
- This is the same as above except for the procedure. The purpose of the procedure is so that someone could reproduce your experimental results. It is probably safe to assume that others can do this without instructions like: "open the bag. pour the bag of contents in the cup. Hold the cup with your left hand and shake it to mix the contents." And then, again do not read all of these instructions. If there was some very important aspect like "you can't use red plates because that attracts bees outside" or something like that, good. Actually, this reminds me of an awesome post someone sent me. This is a post from the Lansey Brother's Blog about science presentations from 6th graders. In this, they show students' science presentation as a list of silly procedures like "open the book and find where it talks about ozone." Check it out, it was funny in a sad kind of way.
- I think the best advice I could give (assuming the project is already completed) for the presentation is to be simple. Try to give a very short overview of the project. Include as many details as you want in your notebook or on your poster, but just keep to the basics. This will leave plenty of time for discussion - the best part. If the judges want to know more detail about a certain aspect, they will ask.
Caution: All judges are not the same. Some do put more emphasis on how the board looks or quality of the presentation.
I think there are a couple of problems evident here. As for the list of supplies, one of the problems with being an elementary school student is that you don't know exactly which supplies are important and which aren't. Obviously, any supplies used in the experiment should be listed as thoroughly as possible (even including the date/time/store where purchased, as well as the brand name, and any other distinguishing characteristics), since even a small variation in a supply may affect the experiment in unexpected ways. Supplies used for making the presentation probably aren't as important, but it may be nice to list those, in case someone else is interested in reproducing certain aspects of the presentation (e.g., future students?).
As for being nervous during the presentation, that's completely understandable. Quite a few kids of that age are quite shy (I was extremely shy as a child, and only became more boisterous later in life.). Plus, given the fact that they're addressing an unfamiliar adult (who has often been presented as an expert in the field) can be quite unpleasant. I've found that it often helps to introduce yourself to the presenter and explain a bit about what you do. Also, it may also help to sit or kneel  so as to be on the same level as the student, rather than standing (Most adults are quite a bit higher than 6th graders, and having to look up at the adult can add to the fear.).
 Yes, our old knees will curse us for this the next day.
While most students have the presentation memorized (and, there's not much that can be done about that), I find the more interesting part the discussion with the student. That's the time when you can ask the student about various parts of the procedure, or to get his thoughts. That can usually go a long way towards determining if the student understood what he did, or if he was only a secondary participant. It can also be enlightening to ask about possible future work.