Pop superstar Britney Spears has taken up yoga and adopted weightloss programme Nutrisystem to help get her back into shape for a planned Las Vegas residency. The Toxic hitmaker and mum-of-two has stripped down to a bikini for the new June (13) issue of America's Shape magazine, in which she shows off her favourite yoga poses and reveals she's trying to tone up to combat the signs of ageing. The 31 year old admits she struggles to accept that her figure is no longer what it once was so she has started to practice yoga. Discussing the changing shape of her backside, she tells the publication, "I like it, then again, I hate it. I'm in my 30s now, so I have to work harder to keep it 'up'." Spears is also overhauling her eating habits with the help of low-calorie meals from Nutrisystem, but she is finding it hard to stick to the healthy food plans. She says, "Dieting is tough... because my specialities are spaghetti, brownies and chocolate chip cookies."