My best fitting experience was with my mum when I was around 21 - she put me in a fitting room in House of Fraser, then got me a selection of bras in different brands, cup sizes, band sizes, and made me try them all on, jump about, put a t shirt on over them to see the shape and so on. I came out of that in a 32 F, having been in a 34 D.
Some years later, when I was clearly beyond a G cup, I went to Bravissimmo, and loved that the fitter did more or less the same as my mum - tape measures were not relevant! And helped me find brands that fit me personally - I find Freya and Panache give me the best shape.
My pet hates are people who, when I say I always shop for bras online, due to lack of availability on the High street, say things like 'debenhams have a range up to a G cup' - I know. That's what I mean when I say High street shops aren't big enough.
Also, fitters in shops who are convinced one of the bras in their shop will fit you, and will shoe horn you in regardless. I've found M&S, Debenhams and most recently Mothercare very guilty of this. For context, I wear a 32J usually, and have been recommende a 38FF (M&S),36G (Debenhams) and my favourite was recently, buying a nursing bra in Mothercare. Knowing my 32J was too tight in the band and the cup, I was sceptical about Mothercare's chances of finding me a good bra, especially with a soft cup. Apparently, I needed a 40F and all my problems would be solved. Needless to say, it was huge round the back, straps taking all the weight, cups trying their best but well beyond design capability.
By contrast, www.nursingbra-shop.co.uk has a lovely man, who, based on my existing bra and concerns helped me find 2 lovely, supportive bras, and was happy to do as many exchanges as necessary.
If a man on the phone can do it, surely a national chain can work it out?
So I no longer go high street, because I know they won't have my size, and will still try to push me to buy - unless I happen upon a Bravissimmo (and even then, they are often very expensive). I order a selection online, at the likes of www.brastop.com and then try them on. Once you know what a well fitting bra feels/looks like, it's easy.