Iman and David Bowie make sure they "have fun" together.The couple - who have 12-year-old daughter Alexandria together - have been married for 20 years and the Somali model says the key to their lasting union is the fact they still enjoy one another's company.She said: "The secrets to a happy marriage are, you have to talk to each other all the time and the other one is have fun together."You have to really like the person and enjoy their company too."Though 57-year-old Iman began her own modelling career when she was just 18 years old, she would be cautious about letting Alexandria follow in her footsteps until she is older.She explained to the new UK issue of Grazia magazine: "I just want her to be a kid, go to school and finish that first."I'm always kind of scared for young girls because it's easy to damage your self-esteem."You could have a great day where you feel beautiful and then Naomi [Campbell] or Linda [Evangelista] walks in and you feel like s**t."Iman credits her own still-youthful appearance to good genes and a strict skincare regime.She explained: "I look like my dad. He's in his 80s but still looks like he's in his 50s. Also, I've been taking care of my skin since my teens."