Port of call: Scenic's Unforgettable Douro cruise

A good reason to book as early as possible is to secure the best air fares

The summer holiday season is just getting started, but before you pack and go, you'd better sit down now and book next year's holiday, if it is going to be a river cruise. This is the launch period for 2016 brochures and discounts end in the next few weeks.

However, early booking fares are not the only reason to act now. Specialist agent Andrew Davies of Go Cruise (gocruise.co.uk) explains: "While it is true that there isn't the dearth of cruises that late bookers were experiencing a couple of years ago, the best cabins at the most popular times are snapped up fast."

Most river cruisers have three decks with balcony cabins on the upper two. These cabins offer views and fresh air and are much more popular than those on the lowest deck with only a high window just above the waterline, which sell last.

Peak months for river cruising in Europe are the milder months of May, June and September, the most pleasant times for sightseeing and to sit on deck, where shade is scarce.

Another good reason to book as early as possible is to secure the best air fares, particularly if you wish to travel on a no-frills airline from a regional airport.

* Book by 31 July to save £300pp on a one-week Enchanting Rhine cruise from Amsterdam to Basel, departing on 14 September 2016 with Ama Waterways (0808 256 8422; amawaterways.co.uk). The early booking fare of £2,799ppp includes flights, excursions and wine with meals.

* Emerald Waterways (0800 270 7017; emeraldwaterways.co.uk) currently has £1,000 off per cabin for bookings made by 31 August. For a balcony cabin on a one-week Delights of the Danube cruise, starting in Budapest and ending in Nuremberg in May, June, or September 2016, fares start at £2,395pp (without balcony from £995pp) including flights, excursions and wine with meals.

* All-inclusive luxury line, Scenic (0808 102 0155; scenic.co.uk) has a discount of £1,500 per couple on its 10-day Unforgettable Douro cruises in Portugal. For example, balcony suites for departures in July 2016 start at £3,290pp and from £3,645pp in June, if booked by 31 July this year. The fare includes flights, open bar, excursions, and butler service.

* A very last-minute offer is with Avalon Waterways (0800 668 1801; avaloncruises.co.uk), which expires at 7pm on Tuesday, to save up to £500pp on cruises sailing in 2016. A 12-day Paris to Prague Central European Experience costs from £2,340pp with flights departing Heathrow on 5 July 2016, hotel accommodation, full board on the ship, and excursions.