I don't write modern historical fiction, but your post reminded me of a writing exercise I once did at university, which you might find helpful. We split into pairs and recorded each other telling a family story. Afterwards, we typed up the other person's transcript into our own story. As most of these family stories were stories that had been passed down (mine was set in the early 1970's, nearly a decade before I was born) they ended up as modern historical fiction.
You could try a variation on the same thing. Interview your Dad, for example. Get him comfortable. Have a few drinks, etc, and get him into the flow - my Dad is the nostalgic type and once he's warmed to a subject will fall back into his old way of speaking. If you transcribe that it should help you to find an authentic voice.
If possible, do the same thing with old films or documentaries set in the era you're interested in writing about. And read as much as you can about the cultural context of the time, until it becomes second nature. I recall an interview with Philippa Gregory which stuck with me - she said that she keeps on researching until the characters start speaking to her. I'm doing a similar thing now with a piece set in the 19th century and it does work (although I've waded through a lot of sources).
Does anyone do this ? How much research do you do and how? Am lucky enough to work somewhere I have access to lots of texts and even some experts who will give me advice in return for cake and tea I love the idea of speaking to people who have actually experienced what I want to write about and creating from this research . I have so many ideas tumbling around my head at the moment. One that has been kicking around for years is based upon the Northern Soul scene..think I may be too late to that party with the film coming out soon. Was talking to my Dad at weekend about his journey over here from Jamaica ...absolutely fascinating. Also considered writing about Mods, punks and the rave scene but since I never actually experienced any of that time I worry about making it authentic?