To find a cheap deal, you really need to do your homework. Flyertalk.com under the mileage tab will usually yield some of the best deals in travel. But there are apps that can do it.
Jetto. You can get one deal for free, but it's $6.99 a month.
Google Flights is another one, which will give you a travel alert.
YapDa will send you an alert when the price dips below a set price.
Check out Johnny's 17 Ways to Find Cheap Flights.