I used to have problems with bras; the wires stabbed and pinched me under the arms, the band rode up and I had to tighten the straps as far as they would go to get the cups to stay up. I started wearing my mum's old nursing bras when I was 10 (I wouldn't look for my own because that was a thing for girls and when I was 10, I didn't want to be a girl) and between then and leaving university, I only went up one band size and one cup size. I hated bra shopping with a passion.
Then, (a couple of years later) on the recommendation of a friend to whom I voiced this dissatisfaction, I went for a proper bra fitting at Bravissimo, which everyone in the UK has probably heard heralded as the Holy Grail of DD+ underwear. My mum was dead-set against it because she's convinced that anyone above a C cup must have had surgery to get there and the idea that I might be bigger than the C I already wore did not go down well. She wanted me to just make do with what I had regardless of how uncomfortable it was.
I've also read a lot of bra blogs and I can recommend Fuller Figure, Fuller Bust [http://fullerfigurefullerbust.com/category/bra-fitting/] (which has already been mentioned), Invest in Your Chest [http://www.investinyourchest.co.uk/bra-fitting-and-lingerie-101] and the Basics of Bra Fitting on Venusian Glow [http://www.venusianglow.com/p/bra-matrix.html#basics]. They're all written primarily for DD+ people but the principles of bra fitting are pretty universal.
I went from wearing a 40C at 23 to wearing a 34FF now, at 26 and the difference is amazing. No more stabbing, no more slippage, boobs stay where I put them rather than trying to do their own thing and all of my clothes fit better.

They always say never to machine wash bras. I've got several with moulded cups and the heat of the wash has done them no favours.
As for underpants, I recently found some which fitted amazingly after years of ones which were too tight in some places and too loose in others. I now own 20 pairs of them and hope they will last me until I find more to replace them.