Old tube works great. You can also apply to other high-wear areas of the bike, such as the top-tube, if worried about it getting damaged.
Don't get one of those stick-on CF ones. They work like poo, I had one on the CX bike and it peeled off after maybe 3 races.
Using the outer chainring is a good idea too, as it loads the RD spring more, as well as keeping the chain further from the stay.
Lots of good suggestions in this thread!