My dogs pose, fairly well at that. Here's a few little tips on getting dogs to pose well, especially when in costume.
Have a treat ready
I mean this. If you want something to turn out good, have a yummy treat in your hand. Or a toy - some dogs enjoy toys more. Teemo likes treats, Raynor likes pettings.
Sit & stay work well, but sometimes I have Teemo beg as well. (Also, long haired me is weird to see)
This gets hard in parks! I usually don't have treats with me either. A lot of this is training, but when possible and starting out, try photographing your pups in your own home or yard. Early photos of Raynor were blurry as she would not sit still, ever.
Don't leave the costume on
I don't usually walk Teemo or Raynor in their outfits - sometimes coats are fine because Raynor actually gets cold and kind of needs one, but hats or scarves drag too easily. I typically carry their outfits/costumes until we start taking photos so that Teemo doesn't trip over his tie (which yes, he has done.)
Don't force it
If my dogs take the costume off more than once, I'm done. If they really don't want to wear it, chances are it's uncomfortable and not fun for them. Coats and jackets they rarely try to get off unless they start playing, but there are some things that never make it to the Internet because they don't love them. Some dogs hate costumes, mine seem to bear it for a treat, and in the case of coats, actually seem to like it. This favorite photo of Raynor I have is actually a fail, she was licking the glasses off and I managed to catch the photo on my second try before giving up because she really hated it. She will let me wrap her in Christmas lights and put a guinea pig on her happily though.