When you are booking a flight, the advice, which I think is now common knowledge is book flights as early as possible. Business travellers will pay high prices and since they usual travel at short notice, airlines hike prices for last minute bookings.
To ensure you get flights that are as cheap as possible check websites of the airlines and comparison sites since prices often as sales often happen and prices change fairly often too. Once you have your destination check all operators to find the best or maybe the most suitable time to travel. Many of you might prefer to pay a bit more than be up at 4am for a 6am flight! Depends on you of course, these times might suit you if you are going on a short break and want to make the most of the day.
There are other ways to further reduce the cost of your flights. Many of the operators offer branded credit cards which have incentives attached to encourage you to sign up. If you are a savvy person you will sign up for the incentive and cancel the credit card if the card doesn't continue to be of benefit to you. Again in you are looking for a flight search about to see what offers are on at the moment. Some offer you free return flights; others allow you to get free upgrades, business class, thank you! Do check the conditions of the offer to ensure they are reasonable and easy enough for you to obtain.
If you are after a cheap flight be aware the cheapest have very strict conditions, ensure you can fly at these times as it is very difficult to get a refund and can be expensive to change dates, it may actually work out cheaper to book a new flight. If you are booking flights as part of a package holiday the operator will be ATOL protected so should the company go bust you will be guaranteed a refund or if you are on your flight you will have a return flight home organised for you.