If the ultra is short, like a 50K, then you probably don't have drop bags to worry about. There you just need to think through your stop as you approach the AS. Get out the electrolyte tablets or take the top off the water bottles. Decide what food you want, if you need Gel's, if you want a sandwich or cookies. Decide if you want to take some food with you. I usually grab a sandwich and a banana as I am leaving. I eat the sandwich as I walk away from the aid station or while I am getting back up to speed. I usually take the banana with me to eat a little later. It is a lot easier to eat a banana on the run than a sandwich. The key is to know what you need and want at the aid station and take the time to follow your plan.
It is important to have planned what you think you will need at each aid station and pack it so it is easy to access. This can be accomplished is any number of ways. I use a lot of zip-lock baggies. I pack the supplies in individually labeled baggies, sometimes baggies within baggies. Some people use various type containers and have items laid out in compartments in a box. If you have a crew member meeting you at drop-bag aid stations they can lay things out on a table or towel. The latter option is the by far the best. Your crew can ask questions to help you think through you needs. However, you cannot always depend on the crew making the aid station. Many aid stations are very remote and difficult to find. Sometimes other things can happen, like another runners crew picks up your bag by mistake with the car keys to your crew's car in the bag. (That happened to Marye Jo at Leadville in 2009 at Turquoise Lake aid station at mile 87.)
All this is important, but all the planning in the world is worth nothing if you don't think ahead and plan your aid station stop as you approach the AS. Look ahead an listen. If you know the course, you will know when your approaching an aid station so you know when to start getting ready. If you are not familiar with the course ask the people you are running with if they know the course. Generally, you can tell as you approach aid stations. You hear cheering or clapping, at night you see lights ahead where there should not be lights. You may know, from studying the race course maps, that an aid station is at a particular location and you can usually tell you are close. Out west, where you can often see for miles, you usually can see the aid stations long before you reach them. I have rarely had an aid station just appear out of nowhere without warning.
The point here is, as you get close to the AS, stop the conversations and start planning. Do a quick inventory of things like electrolyte caps, GU's, etc. If you have a cue sheet, get it out. If you are using an electrolyte additive like NUUN tablets, go ahead and get them out and drop them in the bottle before handing it to the Volunteer. If you are running with a carb mix like Carbo Pro, as I do now, go ahead and get out the next bottle to fill. (I carry four, 10oz Nathan bottles in a belt like the one to the left.) This is one of the most difficult things for me to remember for some reason. I forget to refill them. If the last bottle is empty, just throw it in the drop bag and be done with it.
As you reach the aid station it is a good idea to drink a reasonable amount of the remaining water in you water bottle or hydration pack. That way you will not need to waste time at the aid station drinking water. If you are carrying bottles, go ahead and remove the lid before reaching the aid station. I wear shorts with pockets when racing and when I remove the lid I put it (or them) directly in my pockets. Several times, in the confusion at aid stations I have lost the lid to my bottle. It can be a little frustrating wasting time looking for a lid you lay down without thinking or dropped in the dirt. I have also left an entire water bottle at an aid station and had to go back a couple of hundred yards to get it. If you are wearing a hydration pak, remove it as you approach the aid station. Open up the pack and get the bladder out and open it for the AS volunteers. There are so many different types of closures for hydration packs, sometimes the volunteers have trouble figuring out how to get into it.
More recently, at Rocky Raccoon 100 in February of 2012, I had similar experience at the Dam Nation AS. Dam Nation is located at mile 6.19 and 12.2 of each 20 mile loop. The next aid station is the Park Road AS at mile 15.61 or 3.4 miles away. On the second lap, passing through Dam Nation AS, I was out of Carbo Pro in the bottle I had mixed. I picked up a bottle at the aid station, put it in my belt, went on up to the tent and grabbed a sandwich and what ever else I wanted and headed out. I never filled the bottle of Carbo Pro with water. I figured I could make 6 miles without carbs OK and did not worry about it. I ate a Honey Stinger waffle along the way and figured I would just fill the bottle on the next pass through Dam Nation. Well, I once again forgot to fill the bottle, and again had no carb drink for the next 3.5 miles to park road aid station.
This time I was concerned. It would be 45 minutes or so until I reached the next aid station and I did not have any extra water to mix with the Carbo Pro. That meant no carbs again. I would end up being without my Carbo Pro mix for 10 miles. (1/10 of the entire race.) I did add some of my water to the Carbo Pro, figuring it was better to have a little carbs than none at all. I hoped I would not now run out of water. This shows just how poorly my brain was functioning and I was only 35 miles into the race. It really doesn't matter whether the water you dink has Carbo Pro, Perpetuem, or any thing else in it. It is still water, and still keeps you hydrated. Instead of just mixing it correctly and treating the mix like water, I mixed it way to strong and forced myself to drink it. I did the same thing again at Cheaha. I am not sure which AS it was, I think the second one, but I just did not fill the Carbo Pro bottle, again! I will get this down some day.