Layering T-shirts- (Been done to death, but what can you do.)
The first go to that comes to my mind is the layering t-shirt. Everyone has seen these basic t-shirts. These close fitting tops cover the target without adding too much bulk. Many companies have sprung up making these undershirts available in several styles. They cost about 16.00 each and need to be purchased in quantity so that you have enough of them so that laundry is not too complicated.
Basic Sewing Skills-
My daughter invited me to go swimsuit shopping with her. And because we shopped early we found a great swim suit. This swim suit had one flaw that nearly made my daughter not buy it. The halter straps were previously attached under the arms starting next to the side seam. This strap position caused me to move them forward and change their angle. I changed it for both for comfort on her neck and to reduce potential gapping with movement. Anymore my sewing skills are not used for making clothing, but for making ready made clothing work better for my family.
Ribbed Tanks- If you are dressing down a bit layering with ribbed tanks may be a great cost saving solution. Often they are too low though to cover what needs to be covered if you bend at the waist. In that case some length can be taken out of the strap at the shoulder seam very simply with a bit of matching thread in your sewing machine.
Some Inspiration- Slit in your skirt too high? Add a kick pleat by attaching a hemmed reverse box pleat to the inside of the skirt over the opening. Skirt too short? Add a slip with a band of lace around the bottom to cover your cheesecake. This one is a good one too, if you can find a piece of knit to go with your v-neck top. Or there is this one using stretch lace. Get inspired and get excited!
Modesty is possible even for Prom! If you get excited about modesty and go after it you will find ways.