If sunnier climes beckon this winter, don't leave the job of arranging airline reservations and hotel accommodations to your spouse or secretary. Reason: A good travel agent is likely to do the job better. And if your company buys a lot of travel, your regular agent would be a fool to give you less than top-of-the-line service.

That service can be worth a lot. Among its benefits:

Cheapest available airfare. Hurried or careless travel agents frequently book the flights that appear on their computer screens first. These are often flights on the airline that is providing the computerized booking system. A careful agent will run through a complete listing, set you up with a low fare, and try to get the seating you prefer. If you buy your ticket a week in advance, an agent can sometimes save you up to half the usual fare.

Hotel discounts. Trips booked through a business can often qualify for the so-called corporate rate offered by many hotels, which, although it may not be the lowest rate, might be less than you would otherwise pay for a first-class room. Some travel agents, however, reserve blocks of rooms in major-city hotels. Taking advantage of these blocks can get you a discount of up to 20% off the corporate rate, and it can assure you a room in a hotel that is otherwise sold out.

Special deals. Aggressive travel agents not only package tours, they also cut deals with, say, hotels that are new on the market, thereby getting their clients as much as a 50% discount on their first night there. At least one travel newsletter, Aisle Seat, published by former Five Dollars A Day Tours Inc. partner Elliott Kanbar, serves as a sort of clearinghouse for deals, and may give you or your agent some ideas. One recent issue, for example, listed 10 resorts in such places as Acapulco and Nassau offering group discounts of 25%. Kanbar also offers hotel and restaurant scrip to subscribers. The scrip, which many establishments issue in return for advertising (and which Kanbar then buys from the advertising agencies), can be worth from 25% to 30% off the cost of a room or meal.

A 24-hour-a-day 800 number. "Suppose you have a four- or five-stop, coast-to-coast trip," says Judy Harrington, spokeswoman for Woodside Management Systems Inc., a business travel management company in Boston, "and something comes up when you are in Chicago. Instead of going to L.A., you now want to go to Dallas. Call the 800 number, and they'll cancel your hotel and flight reservations and make new ones." A no-show on one leg of a flight, Harrington points out, leads the airline to cancel the remaining legs, while a no-show for a reserved hotel room leads to a bill. Hence the importance of rearranging the reservations.

A variety of other services. According to Harrington, these services should in clude securing tourist cards and visas, obtaining travelers' checks at no cost, setting up a full rental-car schedule (again, often with discounts), and providing bills that differentiate between personal and business travel.

Since travel agents work on commissions, these services cost you nothing. According to the newsletter Travel Smart, however, there are a few additional services for which you should expect to pay. These include bookings in inns and other small establishments that pay no commission, the cost of more than one or two long-distance phone calls or telexes; and any messenger or delivery costs incurred in last-minute bookings. Still, travel is a highly competitive business, and your agent may be willing to make special allowances. After all, even when you are traveling for pleasure, your business makes you a good customer.