People tell me I am lucky because my dog and cat are always posing for me. I have one advice for them: make your own luck. Roxy seems to always listen to me. Even my cat Mya looks to behave like a dog and sit pretty for me. My secret? Bribe.
Almost every pet will listen to you if you have food in your hand. It helps if you play with them before. Get them to run and play so they can can get rid of some of their energy. Ask them to do some tricks and feed them a treat when they do so. When it is the time for the photo, ask them to sit and hold a treat in your left hand. Make sure they see it. Even better if they can smell it. Remember that it's a pet so don't take 10 minutes to make your photo. You can try it with a toy also. One that makes noise is very good for that. With a cat, it's more difficult. He might climb on you instead of sitting patiently.
Another tip: you can talk to them or make funny sounds that they are not used to hear. You might get a tilted head like that.
On a side note, it might work with kids too. If they don't want to pose, try bribing them. ;-)