Usually, you need to get the discount card in advance, then follow the restrictions as far as reserving in advance and booking on trains during certain times of the day or days of the week.
For areas not served by trains, a car is probably about your only alternative. Remember to take autoroute tolls into account, cause they can add considerably to the cost of a trip, though they cut quite a bit of driving time when compared to sticking to the nationales and departementales.
Another option would be to try ridesharing. Through sites like carpooling.fr, you can find people driving to your destination and book a seat in their car. I used their site and paid just 20 euro to get from Paris to Marseille. You also get to meet some interesting people along the way.
If that doesn't appeal to you, buses tend to be cheaper than trains. I would try companies like Eurolines.
Carpooling is a very nice way to travel, but the very website that started the trend covoiturage.fr is no longer free and i dont know if it will still remain as cheap and easy as it was.
Buying train tickets in advance can help you save a lot of money, with the Prems program of SNCF. But when you cant use your tickets, or you didnt buy your tickets early enough (2, or 3 months) you can still buy/sell it on specialized websites like trocdesprems. One day, I bought an eurostar round trip Paris/London 2 weeks in advance for 65 euros there. Rather cheap. The difficulty is that you have to make an appointment and meet the seller / buyer somewhere, which can be tricky.
Hitchhinking is still possible, and is working okay outside paris if you look neat enough to reassure drivers.
Thanks guys. Just checked out carpooling.fr and it's completely free to use the site so will try it out. It seems to be the cheapest option and will certainly be an adventure. Looking forward to it!
I think it's a different company to the one you mentioned @ihavequestions. Thanks for the tip about selling unused train tickets. I love getting the train but I'm not often organized enough to book that far in advance. The idea of buying a ticket and selling it later if plans fall through is something I have never thought of.