Safety pins are great if you have about a dozen and some kind of ribbon like the ties off the waist of an old dress or shoulder straps.
I have often borrowed the mrs clothes and bras so when I take photos I pin the back to make it fit better around a TB's narrow waist.
If I want rear shots, I pin the front and the opposite side for side stuff.
This is my mrs dress, pinned to fit.
Often TB's have the problem of narrow shoulders and straps tend to fall off, so I tie them close at the back with ribbon or use one of these little numbers;
They cost about $2.00 for a 10 pack, they are made specifically to keep bra straps together for narrow back or crossover strap dresses.
I also find if bras are too lose across the chest you can just tie the back straps together.
For permanently adjusting clothes, I have found it amazingly easy to put clothing on your doll inside out, find the seam, pin it or whip stitch along it to fit, then remove, turn back outside in, check the fit, adjust if needed and sew properly then cut off or fold and stitch the excess flat as I usually do
You may notice the side seam on this blue skirt, I had to taper it in quite a bit for Carly's extreme waist-hip ratio, the excess is just folded back in case I ever want to let it out again, but I think it looks quite acceptable.
These bras back straps have been folded and sewn short so that the shoulder straps come quite close together as well a making the chest tight, as you can see by the shoulder strap positioning, they were made for a rather large woman, but now they fit very snug.
Just remember to do both seams evenly so the zip and any pockets or belt loops stay in correct relation to each other.
Much of my girls stuff comes from op shops, but most are brand names, so if I bugger something up it's no drama.
Pete.