My first bra fitting was my mother taking me to a small department store and choosing me two bras, one beige and one pale blue - apparently white would go grey in the wash so wasn't allowed.
When I first came to England, I heard M&S were the place to go (probably one of those magazine articles that says 'according to M&S 90% of women wear the wrong sized bra'. They measured me, told me I was a C cup, I was so excited as had been up to a DD and they were harder to find. I chose a selection of bras in the size I'd been told I was, went off to the changing room, joining the long queue, only to get to the cubicle and discover there was no way I could fit a boob of mine in a C cup. There was no help in the changing rooms and I was so disappointed with the queuing I went elsewhere.
For a long time I went to John Lewis for fitting, I would choose some pretty bras in styles I liked and bright colours, go to the fitter and say 'I like these but need to check my size', they would check my size and bring me black or white bras in my size. I was never a pretty, coloured bra size for a long while until I one day came in at something like a 34DD and could actually find cheap High St bras in my size.
After having a child and breastfeeding, my boobs just got bigger and back size went down. I am now a 30GG and have been for a few years now.
Worst advice, those shops that answer 'do you have a 30GG' with an insistence that I be measured in case that is not the size that I need, then finally once I've been fitted, concede that it is in fact my size and no, they don't have anything in store that size, when in fact they don't ever do anything in a 30GG. A waste of time. I would prefer the honesty of an answer like 'no, but we do a 32G which is worth trying as different bras fit differently'. Likewise with friends telling me of places that do bras with large cup sizes and sell all sizes, yes, lots of large cup sizes these days but not with small backs.
Best bra fitting (Bravissimo) is having my size checked and the straps on my current bra adjusted to give a better fit before I even try more bras. Also the fitters who explain to use the last hooks on a new bra.
Best tip (Midwife) if you have a bra fitted in a store and it doesn't fit right, take it back, even if you have worn it, it is not acceptable for a fitter to sell something you then wear and fall out of. Mothercare were loathe to accept this at first but their bra fitting was dreadfully inaccurate.