Plan your journey. Local knowledge may prove very beneficial. For an instance if you arrive at Newcastle International airport intending to take a taxi to an address in the Scottish Borders, to which you have never before travelled, will that taxi sat so conveniently on the rank suffice? Shall the driver know where the town you require is let alone an individual address? Will he be prepared to leave the airport to take you there? And what will the cost be? Lots of questions to which you ought to know the answers.
Get various quotes. Competition rules after all, and it may be that a taxi operator not too far removed from along the shortest route between your departure and destination points may provide the best choice. Planning counts for a lot. Careful though, the cheapest price is no guarantee that you will receive the service you deserve to expect, look also for an operator known to deliver what is promised.
Very obvious, Know what your requirements are. Do not book a taxi for three passengers, and show up as a group of five. Do not forget to mention that little diversion you wish to take since one of the group are staying elsewhere, visiting Aunt Bessie's grave, or any other reasoning. The quote you received and accepted was for a pre-planned journey and if you change the plan expect the charging criteria to change.
Last but not least. Be prepared to pay in advance. Your plane, train or bus were all required to be paid in advance, so why not your taxi? After all the other named transport keep to a given time and route that may not always be at your convenience. A taxi however is at the time you specify, the journey you choose, and may have to travel many miles at some ungodly hour to fulfil your contract so payment in advance please.