Europeans should check for last-minute offers; they are often as cheap as (or even cheaper than) any other possibility to travel to Corsica.
If you live in or near France, check for cheap inland flights (Corsica is France).
If you are going to hike as we did, a car is not necessary, it is even an obstacle.
If you use a Corsica ferry during the season, you are charged a supplement of 30 FF. I don't know the purpose and I don't know who receives this, but you'll have to pay it twice. It's called something like "Corsica tax", and you might be charged also when arriving or departing by air.
The SNCM has another trick: If the cheap tickets are sold out, you'll have to buy a more expensive ticket; you won't get it for the same price. So buy your tickets as soon as possible.
If you decide to make use of the SNCF (the French railroad company), look out for discounts! Ask for a "Billet de Séjour" if your journey contains more than 1000km of french railroad, it gives you 25% off the normal price.