When it comes to fitness and health, celebrities rely on their famous friends and expert trainers. Mila Kunis and Ashton Kutcher, Channing Tatum and Jenna Dewan and Nick Lachey and Vanessa Minnillo all workout together as couples! That same method works for besties Kate Upton and Miranda Kerr too! Nothing like a little motivation along the way! As for treating yourself every so often? Trainer Justin Gelband says the key to fitness and diet is mixing it up, and he even advises his A-list clientele to indulge in dark chocolate or ice cream! Karlie Kloss opts for her own cookie creation when she's craving a sweet, and WNBA star Skylar Diggins tells us she chooses pizza to reward herself!