One possibility, depending on how long you are in Paris and whether you want to see other museums, is to get a Paris Museum Pass. (Go to http://en.parismuseumpass.com/ to see what the prices are and which other museums are included, but you don't need to buy it in advance.) The advantage of this is that you can make multiple visits to the Louvre during the period of pass validity, and you don't feel pressured to see everything in one go.
The current prices for the PMP are 42 euros for two days, 56 euros for four, and 69 euros for six. This compares to 12 euros for a single Louvre entry.
If you decide to use a PMP, you can buy it at various sites, including FNAC stores and tourist information desks at both airports. However, one of the easiest ways to get it is at one of the smaller museums that it covers, where there will rarely be long queues.