We don't always want to look like this... |
Basically, you add width to the side princess panel at the bust point (where you are the fullest) and then taper out to nothing at the notches. You do need to make sure to also lengthen the front princess panel, also at the bust point. I always have to shorten the neckline of my tops, so that is why my front princess panel isn't evenly lengthened.
Another option is to muslin your bodice (or tissue fit) and leave the princess seam open between the notches. It will gape open over your bust when you try it on for fit. Measure the widest opening and add half of that measurement to your side princess panel, again lengthening the front princess panel to fit. If you have a lower bust, this is a particularly useful method, as you can easily move the bust point at the same time by adding fullness where you need it, without any regard for where the pattern has it.
My typical method is a combination of these. I make the flat pattern alteration as above, then make my muslin and alter it if I need to using the open seam method. It's taken me a few tweaks to get it just right, but I think it was worth it. Now I have a princess seamed bodice I can count on, and no more Madonna boobs.