I find it hard as I don't understand the dynamics of their family in the way I understand the dynamics between and my own parents. Generally my mum is great anyway, and will always check with me before eg spooning new food into DS; she is also interested in how advice has changed (she and my dad have been really interested since we started using washable nappies, for instance, and doing blw). Crucially, she also trusts me to do a good job, I think. And if she disagrees she can happily hold her tongue, or will gently point the thing out to me. If she does something I don't agree with, I know how to talk to her about it in a way that isn't offensive and which helps her see my point of view.
MIL on the other hand, I find difficult. Partly it's her personality, as she's much more controlling than DM. But partly it's also because I don't know how to point things out to her in a way that doesn't give offence; she's probably the same with me. When she was randomly feeding my 4 month old chocolate buttons, for instance, I didn't know what to say - I asked her not to, and she ignored me. DH was useless as he has never been allowed to stand up to her.
I think it takes time, mutual respect, and adult behaviour on both sides. You've had over 20 years to get to know your own parents, and only a few to get to know your in-laws - time will help to some extent!