Let me first state that I feel that I am in no way qualified to provide any advice or information about alterations. I have no theory, only practice. Over many, many years of owning and operating both a small bust (32AA) and a sewing machine I have worked out my slapdash ways to alter patterns to fit my bust. I am sure anyone with actual knowledge and education will be appalled.
I should say the lack of theory is not wholly my fault. I have tried many books and websites to get more information. Unfortunately, there's not a lot of literature out there about the SBA. Most books carefully explain the FBA, and then say, "And an SBA is the opposite." Totally not helpful. There is one huge exception to the lack of SBA information and that is Shannon's tutorial on Hungry Zombie Couture on how to alter a standard darted blouse for a small bust. It is excellent, clear, well-illustrated, and extremely useful.
I get a fair number of questions about SBAs, so I know I'm not the only one out there looking so I'll share what little I know, with the caveat that it's just stuff I made up, mostly.
I would also like to formally object to the term "Small Bust Adjustment." It does make an easier acronym than, say, "Petite But Well Shaped Bust Adjustment," "Mine Will Stay Perky Longer Than Yours Bust Adjustment," or "I Can Buy Cheap, Cute Bras Bust Adjustment," but those accentuate the positive rather than making us sound inadequate (and my positive needs a lot of accentuating, let me tell you).
I generally choose patterns that are easy to alter with my bust in mind. Despite Shannon's fabulous tutorial, it is very rare for me to make a standard darted blouse because of the difficulty in altering it to fit the bust. Other styles are much easier, and at the top of the list is princess seams. I looooove princess seams (as if you couldn't tell from this, this, this, this, etc. ad infinitum). It makes no difference to me whether it is shoulder or armscye princess, I love them all. Shoulder is less common, and I wonder if that's because it's less curvy--which means it might be better for us. I haven't tried out enough shoulder princess patterns to tell (because there are so few).
To alter a princess seam all you have to do is shave some width off the bust curve on the side front pieces. Super easy.
If it's not princess, I usually go for something in an empire line with a bodice that hits right under the bust. I generally reduce the width of the gathered section or of the dart, and if the bottom of the front bodice piece is very rounded I flatten that out a little, as I did here for Vogue 8386; here's a comparison between the final altered bodice and the original pattern. If you don't flatten, you get what I call "Empty Bag Syndrome," where the extra fabric spills out over the underbust seam; it's like the opposite of the pencil test:
I have a short distance between my bust and my shoulder; I don't know if this is related to my small bust or my frame, but I suspect it's a little of both. I often shorten the front strap, usually taking in a little more at the neck edge than the shoulder edge (thus shortening the front neckline, which prevents gape). You can see that in the photo above for Vogue 8386, and here for New Look 6394. This is kind of a touchy alteration because it can create serious drag lines running from neck edge at shoulder to the armpit; this is especially a problem for styles with sleeves. I haven't found a way to deal with that problem and only go with the slanted front shoulder if there's no other way to get rid of the gape. I'd rather have drag lines than gape.
Wrap styles were always off limits to me until I discovered how to alter them by realizing that the problem really was as simple as the crossover neckline being too long. By taking out some length, I opened up a whole world of new styles. I am petite but have a long torso; Cidell was startled to find that our torso measurements are nearly identical though she is several inches taller than me (and well into the average height for which patterns are drafted). So I think this is not related to my shortness and the principle applies to SBAers regardless of height. For crossover necks, I take out a wedge of width at a 90 degree angle to the neckline. Don't forget to alter the facing if it has one, as with Vogue 8379 (though frankly I didn't love the facing on this pattern and will leave it off next time I make it). This one is still a work in progress because the angle of the marked pleats is wrong for my bust, I just haven't figured out which way to aim them:
For McCall 5314, a woven wrap, the line got too distorted when I tried to take all the length out at one go, so I shortened at about 1/3 and 2/3 of the neckline distance:
I mean seriously! Check out how ridiculous that alteration is! I took out about three inches of length! I really don't know who that *wouldn't* gape on. But the pattern looks fab on me now and is as snug as a bug in a rug along that wrap. No gape at all.
Sometimes you need to alter the pattern along more than one axis, as here for Simplicity 3775. I shortened the V neck/mock wrap to avoid gape, and narrowed the width of the gathering. When it comes to gathering, I also find it better to gather more toward the center front than to follow the markings. I don't know if it's my body in particular or general to a small bust to be more centered on the chest, but my girls seem closer together than patterns are drafted for.
For other miscellaneous styles, always, always, always narrow the width of a bust dart, whether it is vertical (a fisheye dart below the bust) or horizontal (a traditional dart to the side of the bust). It is invariably a mistake not to do this, and I made this mistake recently! Before I had any idea what I was doing I tried to deal with bad bust darts on the garment by taking in more fabric to try to reduce the bagginess. While that sounds like it will work, making a dart bigger makes it worse by increasing the difference in circumference between the bust and below the bust! It is essential to alter the pattern before cutting so there is less fabric to begin with.
You might also need to reduce the length of a bust dart. It's worth it to do a quick tissue fit and mark your bust point (i.e., center of your bust, i.e., nipple) on the pattern piece. If the dart apex is going to overshoot your bust point it needs to be shortened.
This post is still a work in progress, because I think there might be something to shortening armscyes in some patterns. I just made BWOF 02-2008-103 and it looks pretty bad under the arms and across to the bust. I think a higher armscye would deal with that, and I think it has something to do with a small bust (since a larger bust would displace that extra fabric and prevent the bagginess under the arm).
Luckily, a small bust is usually fairly high so we don't have to worry about lowering the bust point. That's some consolation, right?