Here is a walk-through with some of the shortcuts that I like to use.
Normally, you'd enter text and inline formulas using the cell style
"Text", and when you're ready to enter a displayed equation you have to start a new cell.
At that point, you would go to the menu and select, e.g., the style
"DisplayFormulaNumbered" with the mouse. Having to take your hands off the keyboard for this purpose inevitably slows you down. But this can be avoided if you make use of the CMD-0 shortcut for
Format > Style > Other...:
At this point, I select
DisplayFormulaNumbered, and this selection will be remembered next time you do CMD-0 to insert further displayed equations, so you can dismiss the dialog without using the mouse by pressing Return. It's even remembered across Mathematica sessions. This is the fastest way to get into the
DisplayFormulaNumbered environment in a new cell.
Usually I now enter the equation in
TraditionalForm. The default format type for displayed equations is not
TraditionalForm, but you get that format either by converting an existing inline equation to
DisplayFormulaNumbered (as mentioned by the OP), or by pasting a
TraditionalForm expression, or by doing Cmd-Shift-T on whatever you type in the new
Another useful trick for numbered equations is the shortcut for entering equation references described in Albert Retey's answer to "Creating a cross referencing shortcut."
If you now want to change the margins or alignment of the
DisplayFormulaNumbered cell, that can be done by selecting
Format > Edit Stylesheet... and choosing the style
DisplayFormulaNumbered from the popup menu. Then you can highlight the template cell that appears, and change (e.g.)
Format > Text Alignment > Align Center.