Jessica Simpson was allowed to keep the personal trainer she shared with on/off boyfriend John Mayer after they split.

The 'Dukes of Hazzard' star was introduced to trainer Harley Pasternak by the country singer just over two months ago, and while she is no longer seeing Mayer, Jessica is still working out with Pasternak.

The singer-and-actress is said to have dropped at least two dress sizes and lost almost 20lbs in eight weeks.

Pasternak told the New York Daily News newspaper:

I don't use a tape measure or the scales. Weighing and measuring don't create a lifestyle change. It is always defeating if you've been doing a lot of exercise, then you step on the scale and you're no lighter. It should be about how you look and how you feel.

The exercise sessions can be as short as 25 minutes or as long as 55 minutes.

Jessica visits the trainer five times a week and her routine includes step-ups, side bends, crunches, dumbbell rows and the occasional use of an exercise ball.

She is also on Pasternak's '5 Factor Diet', also favored by other celebrities including Vanessa Williams, Katherine Heigl and Eva Mendes.

He said:

It is very simple and not at all wacky or controversial.

I am the voice of moderation and there is no carb or calorie-cutting. It is very sustainable and painless.

Jessica, 26, is allowed one day off when she can eat anything she likes, but Pasternak claims his regime means she won't be tempted to use these days to binge on fast food.

He said:

The plan promotes an entire lifestyle change. After a while my clients get sick if they splurge on unhealthy foods.