Let them know your key requirements.
Let them know what is important to you about the deal you have found.
Try to use firms belonging to trade organisations offering financial protection for consumers, such as ABTA, ATOL etc.
If you have a credit card and your holiday costs more than £100, use it. Section 75 of the Consumer Credit Act 1974 makes credit card companies 'equally liable' for breaches of contract and they may be pursued if the holiday company has ceased trading.
It is also important to know your rights. When you book a holiday, you enter into a contract, usually with a tour operator. This means the law gives you automatic statutory rights and you are entitled to receive the holiday or flight on exactly the same terms as advertised, including photographs, room type, board basis, etc.