Like every other business, the travel and tourism industry is affected by the basic economics of supply and demand. Therefore if you book your holiday 11 months before you intend on travelling, there will be far less demand and therefore much cheaper prices for you. Flights will have already been released so there are plenty of cheap seats available on the aeroplane as well.
If you cannot or have not booked early, then consider booking late. As with booking early, the same economical factors of supply and demand still apply. There is less demand for last-minute holidays due to the fact that people on the whole need time to plan. Be organised, bag a last minute deal and be overwhelmed by how much you could save on your holiday.
Use voucher codes
Travel agents such as thomascook.com often release promotion discount codes in order to encourage customers to book their holiday through themselves. Using thomascook.com discount codes can save you anywhere from £100 to £800 on your holiday - and sometimes even more than that. It may seem like they are giving money away but if this means that they take the business from their competitors then it's well worth the investment of offering the discount.
Choose your days and times carefully
You will be surprised at exactly how much money you can save by flying on a Tuesday or Wednesday as opposed to a Friday or Saturday. Likewise if you fly at 2am it will be considerably cheaper than flying at 3pm. Getting up very late or very early to go on holiday adds to the excitement as well. If you have children then you can't really avoid paying the premium for going on holiday in July and August, but some travel agents offer free children's places and money off for adults as part of the governments Every Lesson Counts campaign for anti-truancy.
Consider buses, local restaurants and cheap hotel chains
In the UK we have many budget hotels such as Travelodge and Premier Inn. Other countries also have their own versions of budget hotel chains, including these two. Consider staying in one of these as opposed to a tourist-trap of a hotel. Budget hotels are available in all of the best locations and you save a lot of money on the no-frills service that they have to offer. You should also research your destination so that you can use buses instead of taxis, and visit the same restaurants that the locals use. Not only is it more cultural than the tourist-trap restaurants but it will also save you a lot of money over the course of your holiday.
If you have any more suggestions then please feel free to suggest them to me so that I can share them with the world!