How to avoid excess baggage fees with Ryanair and Air Corsica?

It's not really possible to completely avoid the extra charges with these airlines, they built their whole business model on it. What you can do is travel super light with just a carry-on bag, check their exact size requirements before you leave (they're weird and different between the two), and take advantage of periods when they run promos on baggage - it's pretty common for them to offer them free or cut the price in half. If you travel often, a loyalty card can also give you an included bag, so it's worth checking out.