Maximum effect is acheived with the wax, scrape, brush, overlay sequence with no skiing before racing. The hard wax (flouro or otherwise) from hot waxing will be very effective for a while, a day, two days; depends on the wax, the skiing, the prior ski prep, etc. The overlays are good for just one run, maybe, two, but in serious racing you'd apply your overlays for each run. For your racing, I'd say a hot wax each night would serve you well. If you are in a tough class, or are fighting for a tenth or two, I'd apply the overlays each race run to get the ultimate performance. If you don't have access to hotwaxing your skis for day 2, I'd definitely use overlays to be competitive.
I use TOKO products. For an effective overlay, I'd suggest using Jet Stream. Red and/or Yellow depending on the snow temps you expect to see. For on the slope, I'd take a combo brass/nylon brush, the Jet Stream and a cork. You'll have scraped the skis clean and brushed them thoroughly before you get to the slopes. Lightly brass brush the skis at the start to remove any hard wax that will have been squeezed out of the cold ski since the last brushing, wiping off any dust. Rub on the Jet Stream to cover the base from tip to tail for at least 50% coverage. Rub in with the cork. This will spread the overlay to cover the entire ski in JS. Try not to rub off, but rub in to the ski. Then use the nylon brush from tip to tail to finish the treatment. Set the skis aside and walk them to the start so they have maximum 'juice' when you are in the start.
Warm weather will create problems that you can easily avoid with a little preparation. Consider ice on the sidewalls and topsheets. Keep your skis in a place where they won't get hot from the sun. Hot skis placed in the snow will eventually develop ice on the sidewalls which creates considerable drag. A clean topsheet will insure that you don't have a pound of snow on a tip or tail to affect the ski. Use a plastic scraper to clear off the ice. You can also use the scraper to clean the soles of your boots. Another trick if you can't find shade, is to bury your skis in powder leaving just the binding exposed to find them again. Wipe all snow off when you pull them out of the snow. You can bury the skis before or after juicing. If after juicing, just don't slide them around until you put them on right before your start.
Good luck racing at Nationals.
BTW, I presume you are stripping down to a suit for the races. If not, don't worry about the wax.