Unwanted opinions and advice

How do you deal with unwanted opinions and advice. Especially if it's from family members? 
I'm having trouble thinking how to handle "you don't need to do this or lift that, you're not suppose to eat that! Don't lay on your stomach!". I know my limits and you're not my doctor. I'm tired of getting lectures on what I can't do. 
It's my first child and in laws are already looking to buy a crib and car seat for them as if Id allow the baby to stay the night or to be taken places. 
Then my mom complaining about me wanting to breastfeed because she won't be able to feed the baby! 
Please tell me this gets better or is this something I have to lay the law down and do it fast? I heard it gets worse once the baby gets here.