How do you know when to purchanse plane tickets?

I am traveling from Indy to Cancun the first week of April. I've been watching ticket prices over the weekend, and was told that prices went DOWN during the week, but they actually went up! How do you know when is the best time to buy?? I am trying to do the best job of a budget I can and don't want to spend unnecessary cash on the tix. Should I try to wait it out and buy closer, or do I need to hurry up and get them now before they go up more?