In my experience (only a few years so far), the designers and management who are getting the bad news don't like it (obviously). I find that if I listen to them, let them get it off their chest, then point out 'that the data gathered is what it is, and yes the test was conducted correctly, so what can we do to improve the item?'
This normally takes a couple of goes to get the info into them. The hardest part is convincing them that the test was done correctly (lots of parameters recorded and compared to the intended test conditions usually does it).
In the case of crew evaluations (HMI etc), the designer has to be convinced that the crew know what they are talking about, which can be quite difficult. Sometimes it even pays to get the designer to the aircraft, dress him up (gloves, helmet etc) and get him to try and use the items in question (not always practical).
I would say the best way to deliver the bad news is clearly, concisely (as with any data), have enough facts/data to back you up, and, if possible, try to think of ways to improve the bad news. Ideas for the redesign, relocation or further testing etc to try to end on a positive note and get them thinking positively (and save time), not all black and negative.
Even so, there are some designers/managers who will over ride us, and say 'too late to redesign, too costly' etc. Then the customer comes along and says the same thing and suddenly they all start panicking like it was a new thing. I've tried to get the info across, which I think is part of my job, but some people just don't listen.
Above all, always remain calm and professional, designers don't like bad news about 'their baby' (who does), and can sometimes get a little arsey, try not to get aggravated when someone looks like they're doubting your work/abilities, they're just looking for an excuse to save time/money/their jobs.
Patience and perseverance usually work for me.
Overall, I prefer to work with computers than managers/designers, as you only have to punch the information into a computer once. (though, as said above, try not to punch the designer!)