Black flies in the Adirondacks are at their worst in the springtime. Moist, warm weather brings them out, and when the hatch is on thick they can be hard to tolerate. Of course the Lake Placid Conclave is held during black fly season, so it helps to be prepared. Some days we'll be lucky: cool, overcast, breezy, and/or rainy days will keep the fly levels tolerable. Hot, dry days will keep them down. It's worth noting that at the last few LP Claves the flies weren't much of an issue. They don't call me "Rainmaker" for nothing....In any case, here are a few tips that will help. They may not help everyone, and some may disagree with a few. None of these tips are a magic bullet for fighting black flies. They are part of a successful strategy for dealing with them, and employing several of them simultaneously will help.
Watch your diet. Sweets, meats, etc in your diet affect your body's odors. All those things nutritionists tell us to avoid are things you should avoid if you don't want to attract black flies. Don't give up all the good stuff, mind you, just be aware that it comes at a cost.
Wear long-sleeved clothing. Elastic at the cuffs can keep the pesky buggers out. Some report that this results in a circular ring of bug bites around your wrist, but that's better than having them crawl around in your shirt, eh?
Avoid blue clothing, it's the black fly equivalent of waving a red flag at a bull.
Wear a broad-rimmed hat, put repellent on the brim.
Avoid Avon bug repellent lotions. No, I'm not kidding. They work very well for a few people, and apparently work well for mosquitos, but there are many, many reports that e.g. Skin So Soft just tenderizes your skin and makes it taste better so the flies have an easier job dining on you.
If you have a favorite garlic/vitamin regimen for repellancy, go for it. I prefer garlic. Others load up on B vitamins. Just don't rely on it as your sole source of repellancy.
Cigars are great for keeping the buggers out of your face. Especially if the cigars are lit and you're smoking them.
If you can fish while wearing a headnet, go to your nearest Army/Navy surplus store and grab one for $3-4. They can be useful, though it's hard--and potentially dangerous--to wear them and smoke at the same time.
Most importantly....DEET. I repeat, DEET. Make sure you have a spray bottle of some product which contains DEET. It works better than other black fly repellent currently on the market. It also, unfortunately, melts PVC fly lines, which is why I recommend a spray bottle to keep the stuff off of your hands.