Congrats on getting your first harvest gig! Echoing what some of the others have said:

-Be on time. Might be tough after a few 12+ hour days, but just be on time. I assure you above all the winery just wants someone who is dependable. -Listen. Each winery might have their own slightly different way of doing things, but there's probably a reason for it. -Ask questions/over-communicate. Make double-sure you're confident about what you're doing... it'll be too late once you've racked a tank full of Syrah into a tank of Riesling. For all the advances in wine, the varietal extractor has yet to be invented. -If all else fails, clean. You'll be quite busy, but when there's down-time, find something to clean to make yourself useful. Squeegee floors, clean around tank doors, empty the nasty drain grates... do something. Anything but stand around with your hands in your pocket. If you're not sure what you should be doing - just ask. -Anticipate the next step. This will come as harvest goes along, but this is where you can make yourself valuable. From one day to the next, you might be running the press, racking, barreling, maybe even helping the lab inoculate. Get familiar with each procedure (bring a notepad and take notes if you need to) and work out ways to maximize each day. -Get yourself some good boots unless you want to go home with wet feet every day. Neoprene is your friend. I used to rock steel-toe boots from Xtra-Tuff and loved them, though I've heard the quality has gone downhill lately. - Lastly, have fun. Harvest is an awesome and crazy time of year. You'll be tired, cranky, and sore... but there's nothing quite like it!