TwoDogs
Boxer Insane
Alot of pups sit in a splayed "frog" sit. It's because they don't have strength in their core muscles yet. Most pups grow out of it as they mature and develop strength in their different muscle areas. You can do exercises to increase core strength. I do some of these with my dogs to build strength for flyball and agility.
http://www.cleanrun.com/images/features/StrengtheningExercisesForAgility1.pdf
http://www.cleanrun.com/images/features/StrengtheningExercisesForAgility2.pdf
I also have my dogs sit on an inflatable balance cusion. It makes for a nice tight, tucked sit and a dog that doesn't adjust its position during that 3 minute obedience sit stay.
http://www.cleanrun.com/images/features/StrengtheningExercisesForAgility1.pdf
http://www.cleanrun.com/images/features/StrengtheningExercisesForAgility2.pdf
I also have my dogs sit on an inflatable balance cusion. It makes for a nice tight, tucked sit and a dog that doesn't adjust its position during that 3 minute obedience sit stay.