If you want to explicitly set the tab ordering for elements in your form, the following attached property is supposed to help:
<Control KeyboardNavigation.TabIndex="0" ... />
I say "supposed to help" as I haven't found it very reliable though I probably need to read more about how it is intended to be used. I only post this half baked answer because no one else mentioned this property.
Note that in Win RT, the property is just TabIndex="0".
Another alternative that has worked for me in the past is to simply remove all explicit TabIndex statements, and let the controls use the order that they're declared in XAML work their magic.
This, of course, may require you to reorder your controls. But this is a simple copy-paste operation.