Don't be fooled by lyrical descriptions:
"The hotel is set in the countryside immediately across the road from the beautiful beach." Sounds tranquil? But what about that road — is it a country lane or a six-lane highway? Countryside doesn't always mean quiet — dogs barking, donkeys braying — nor pleasantly smelling. And a "beautiful" beach — it doesn't say it's safe for swimming, nor even accessible.

Don't make assumptions:
If the brochure doesn't actually say the hotel has air-conditioning (or a pool, or bar or whatever else is important to you) then it probably doesn't have it — even if every other hotel on the page (or indeed whole brochure) does have it. If in doubt, ask the tour operator. If you get no satisfactory answer and the point is important to you, don't book.

Take the relevant pages of the brochure on holiday with you:
There's no way you'll remember all that small print.

you may be entitled to reject the holiday and require the return of any money you have paid (and you may be entitled to compensation if you have to spend more to buy a similar holiday), or

you can accept the change and go on the (different) holiday, sometimes coupled with compensation as per the booking conditions.

Do so as soon as you become aware of a problem.

Complain politely but firmly.

Put your complaint in writing — to the rep and to the tour operator, using its complaints form or procedure if there is one — and keep a copy.

Keep a record of your complaints and any suggestions made by the rep.

Don't reject a solution by the rep if it is the best they can do, even if it is not entirely satisfactory. However, if you accept a partial solution to the problem, make it clear that you are still not entirely happy.

Keep receipts for all extra expenses incurred because of the problem.

Gather evidence — photographs, movies.

Enlist witnesses and get their phone numbers and/or addresses in the UK.

If a problem affects other holidaymakers as well, combine resources for a joint complaint (both at the resort and on return to the UK).

Flight delays:
You may receive surprisingly little compensation for these. New statutory regulations do affect your right to compensation for delays. For long delays you may be able to claim on your insurance.

Lost luggage:
The tour operator will usually not take responsibility. This leaves a claim on either the airline or your insurance (but not both).

The accommodation is not as described in the brochure:
Then it's breach of contract and you can claim compensation. But make sure you complain to the rep and try to get the problem rectified.

The accommodation you booked is not available:
Although your contract may specify particular accommodation, the booking conditions may allow the tour operator to substitute accommodation in the event of overbooking. However, the alternative accommodation must be of the same or higher quality or the contract will have been breached.

Standard of accommodation:
Even if not specifically mentioned in the brochure, you are entitled to accommodation of a reasonable standard of cleanliness and quality in keeping with the cost and type of holiday. This is a rather loose concept, but if you are not happy you should complain to the rep.

Safety:
The tour operator is responsible for the safety of the accommodation.

Expenses:
You can claim your out-of-pocket expenses for everything you had to pay that you would not have done if you had received the holiday you booked.

Loss of enjoyment:
In most suits for breach of contract you can only claim for material loss, and cannot get damages for any distress or disappointment caused by the other party not fulfilling its obligations. However, with a holiday the whole point of the contract was to provide pleasure. When things go seriously wrong with holidays you can make a claim for damages that takes account of the disappointment, and even inconvenience, you suffer as a result — although of course loss of enjoyment is a very subjective matter and open to interpretation.

Pain and suffering:
If you suffered serious illness or injury due negligence on the part of the tour operator or hotel this is an entirely different matter and you really do need to consult a solicitor. Mark Goldstein will be happy to advise.