What styles of clothing are flattering when you have a double mastectomy and will not do reconstruction? Any guiding principles?

There are several special types of bras and breast prosthesis which will simulate breast shape and contour for patients who choose not to undergo breast reconstruction. You should look into those. In some cases your insurance will even cover it.

Thanks, but foobs are not for me.  I have been fitted for a pair but don't want to have fake breasts. Am wondering about styles that de-accentuate chest shape. 

There are many stytles of clothing that will "de-accentuate" your chest area.

Small busts look great in high or slashed-neck T-shirts, and vests with cut-away sleeves. An alternative is a round neck with short capped sleeves, which will draw attention to your arms and shoulders rather than your chest.
Detailing is also good like ruffles, appliques or a design. Wear bulkier fabrics that hang rather than cling.

Wearing jeans that have a slight flare at the leg will make your waist look smaller and even out the chest area. Belted blouses can puff up the chest area and not be clinging.

The use of large scarves draped across the front are good too, and they are very "in style" now.

And you can wear backless without a bra! Lucky you!  

Peace & good luck! 

Savvy Sister, you ROCK. Once I decode all your instructions (you can see that I am fashion-challenged!), I will follow them. Thank you so much for your thoughtful response. Yesterday I did Bikram yoga with no top at all, and THAT felt very freeing. Don't think it would work outside a yoga room though --- :-). Thanks again for your kindness.

Your welcome. Please let me know if you need any other help. I'm sure you're not the only one that does yoga "in their birthday suit"!