Communicate effectively. Some skippers want a constant stream of information being fed to them about puffs, waves, distance and angle to the mark, sail trim, tactical suggestions (when to tack, when to stand on), info on competitors; When they tack, how they are pointing, their relative speed, do they have better wind etc. Some skippers want you to just be quiet.
Don't wander around the deck. Keep movements to a minimum and keep your weight in the "right" spot. Heel the boat according to conditions, move weight fore and aft as directed by the helmsman. Hike HARD. Basically, make all your movements pre-calculated and mak them efficient.
With the above in mind, be especially careful about the foredeck. Adding weight upfront kills speed in a hurry. If you must step on the foredeck, have a thought out plan in your mind. For example, don't stomp out on the foredeck to adjust the spin gear and then realize you've forgotten to give yourself enough slack, forcing you to stomp back to release it.
When hoisting, hoist for all you're worth. No matter how fast you hoist it, you could still do it faster. Don't worry, the cockpit crew will remind you.
When dousing, douse for all you're worth. Get the foot of the spinnaker onboard and pull it in as fast as humanly possible. It still won't be fast enough. Don't worry, the cockpit crew will remind you.
If you're on foredeck, run your tapes on the spinnaker. Don't trust it was done correctly by the last guy. Run your own spinnaker lines/sheets. Don't trust they were rigged properly by the last guy. Be very loud and clear when you need slack on the sheets and particularly when the pole is made. Even though you are foredeck crew, stay off the foredeck. Don't worry though, if you've made any mistakes, the cockpit crew will remind you. Make sure you help out your helmsman at the start; Be sure to work out the signals ahead of time, but you likely want to give them signals for how many boat lengths you are away from the line as well as if there are any boats underneath you that could force you up or over early.
If you're in the cockpit, talk to the skipper. Give them feedback on windstrength, trim constantly.
Jib trimmer? Be quick on the jib, but not too soon. On a 26', you should be able to get it trimmed fast enough to not need the winch. Communicate well with the skipper, on my main crew, the ready symbol is given by the helmsman, but the timing of the actual tack is based off the jib trimmers action of removing the winch handle. As soon as they are out, we are going. Keep the spare jibsheet wrapped loosely on the windward winch and keep the slack out of the windward line. At the start of a race, keep the winch handles out of the winches and have one person do both sides of the cockpit... this may not be necessary if you don't spin on a dime, but it helps if you're dicing it up at the start. Don't forget to remind the foredeck crew about their mistakes, they need all the advice and backseat driving you can give them.
Main? Follow the helmsman, don't wait for him to tell you. Look up. Wear sunscreen - your ball cap doesn't protect your face while you're constantly staring straight up at the tell tales, camber, twist etc. Hike out! I don't know if the helmsman also does main on your boat, but lots of main trimmers don't bother hiking after they have the sails set. Also, once you hear the helmsman or jib trimmer remind the foredeck crew about their mistakes, be sure to chime in. A second or third voice really helps the situation.
I'm not qualified to give advice as a helmsman... so take the following for what it's worth.
Don't hit anything. Sail to your target speeds. If you don't have target speeds, find them. Counter-intuitively, if the wind drops head up to bleed off speed as your target speed is lower now - this gains you a bit to windward. If the wind increases, bear-off a touch to pick up speed to meet your targets - you will be faster to your target speed and then able to get back up onto your angle faster. Watch for waves, they stop you dead. Listen to your crew. Talk to your crew. Don't bang the corners unless you need a hail-mary. Take the long tack first. ok, so much more, but since you won't be at the helm, just remember, to remind your foredeck crew of their mistakes, you're the helmsman, it's your job.
Lastly, bring cookies. They make up for any mistakes.