There's nothing sacred about New Year's or Mondays for starting to incorporate better eating and exercise habits into your life. It's a lifetime commitment that will be just as important on a Friday in July as it is right now.

``If you get specific about times of the year or times of the week, you're setting yourself up for failure,'' said Franca B. Alphin, director of nutrition at the center.

``People often look at Monday as a fresh start and Friday as a failure if they haven't done well, and then weekends are a binge time, and then we start again on Monday.''

A more practical approach would be to plan each morning how best to monitor your eating for the day. Realize that you'll have your good days and your bad days and don't give up if you falter, said Alphin, who was in Seattle this week promoting ``The Duke University Medical Center Book of Diet and Fitness'' (Fawcett Columbine Ballantine Books, 1991, $19.95).

You may have a set time when you start trying to lose weight by consuming fewer calories than you burn. But the basic truth is that once you reach your goal weight, you still have to monitor because you'll never be able to consume more calories than you burn without gaining weight.

Trying to be perfect is often overwhelming. Trying to do better is easier to accomplish.

``A lot of people who have weight problems are extremely successful in other aspects of their lives,'' Alphin said. ``Food is the one area they can't control. They tend to be perfectionists and they feel that one bad day constitutes failure.

``They need to think, `OK, I've had a bad day, I need to start again the next day.' ''

And that's where the Duke University book differs from many other books that focus on diet and exercise. It may not offer any revelations, but it does offer a comprehensive look at what constitutes proper nutrition and exercise, as well as common-sense views of eating behavior, hidden calories and making lifetime changes.

Based on examples gleaned from the Duke University Diet and Fitness Center programs, where people go to live for a week to develop good exercise and nutritional habits, the book recognizes human frailty and offers ways to hold your own hand during the inevitable setbacks.

The tenets for a healthful diet follow national guidelines:

-- Reduce total fat, saturated fat and cholesterol intake.

-- Use salt in moderation.

-- Eat more complex carbohydrates: grains, vegetables and dried beans.

-- Eat fewer simple sugars: table sugar, cakes and candies.

-- Eat more dietary fiber.

-- Reduce alcohol consumption.

The book gives some ideas on how to accomplish that.

There are charts that show how many calories you'll likely need in a day based on your height and fitness level. The book has recipes and menu suggestions that allow you to cut down to 1,000 to 1,200 calories a day until you reach your goal weight, while still making good nutritional choices.

It points out where the traps are in eating patterns and emotions, going out to dinner, and even in deciphering food labels.

People in the United States are unusually well-educated about food and food choices, Alphin said, but that doesn't always help them make the right choices. The emphasis on fast food in the past decade has made that especially true, she said, although some frozen food products and some fast-food chains are responding to consumer demand for more healthful food.

``As a nation, we understand the message, but it's a matter of taking the initiative to make a change,'' Alphin said.

``If you have to choose between what appears to be a bland diet - broiled fish without any butter, vegetables that are steamed, limiting your intake to maybe one slice of bread or a roll - or sitting down and allowing yourself everything you want, there's no doubt it takes an effort.''

But you don't have to swear off pizza or chocolate for the rest of your life; you just have to realize you can't eat either with abandon.

Diet restrictions alone won't do it. You have to include exercise, but the book makes the point that it doesn't require Olympic-caliber training to make a difference.

People in their 80s and 90s say they have increased energy and feel more alert from moderate exercise. Walkers, once scorned, have the lowest dropout rate of any exercise group.

``Not only does exercise make you feel better and help give you tone, but it allows you a little more leeway if you want to sneak that extra treat once in a while,'' Alphin said.

Perhaps most important, if you weren't born looking like Jane Fonda or the male equivalent, even if you trim down you probably can't expect to make a fortune starring on aerobics tapes. You need to set achievable goals.

``We're all proportioned differently,'' Alphin said. ``People will see improvements, but if you have a pear-shaped figure, you'll always have a pear-shaped figure.''