More than 10 people will get you a discount with most carriers.
Most airlines will tell you when they have deals going on but not what deals they will be. Example is Southwest posts their sales 8AM CST Tuesdays and if you want to take advantage of that sale you must book by Midnight CST that Thursday. If you book with them, then they offer a price drop, you have to call them but they will refund you the difference as a credit for any trip any where for any one for 12 months from original date of purchase (you can give it to a friend to use or whatever). Book for $600, drops to $500- you have $100 credit with them.
Most of them have dropped their fees for canceling as well. I'm going to Vegas, found a flight 2/3 the price at a closer airport so canceled my original flight with 1 carrier so I have the full price I paid credit with them and booked with another carrier. Sure- the 1st carrier keeps my money until I use the credit but plan on using that credit for a trip in May for my family so no big deal.
Most airlines claim they have the best rates on their websites vs orbitz, expedia, etc so check with one of them to narrow your flight choices and then check with the actual airline website for a better rate.
I know that didn't really answer when is the best time to buy but just general info. I watch fares for about a week to see what's "normal" and then book when I see a deal, the sooner the better since once the plane starts filling up, the price goes up as well until the last day or so. Then they offer last minute pricing deals but you don't really want to risk doing a hail mary if you have to be some place; those are more the weekend warrior deals.