Problems with my 3 month old Golden Retriever

Question
I'm sure you get these questions all the time... but I'm curious to see if you can help me... My puppy has been with me since he was 5 weeks old. He was calm, never cried or whined, has quickly learned to sit, down, lay down, hand, and fetching. But the past couple of weeks, I have noticed his biting has gotten harder and the more I tell him no, the more he does it. So I read somewhere that if he starts to nip, you lightly grab his muzzle and hold it closed and tell him NO. I have been doing that, but he doesn't seem to get it... he thinks I'm playing, and as soon as I let my hand off, he does it again. I'm sure he's just playing, but it has gotten to be too much. The other night he bit my hand and punctured the skin a bit... I know he doesn't mean to hurt me, but I don't know how I can make him understand that he can't bite.

If you could help me out, I'd really appreciate it! :) Thank you!

Answer
This is a big problem.  Young Labs, which I know best, and other puppies tend to very bad about
biting.  You see a litter of them, and all the ones that are awake are biting
another one or themselves.  I am not even sure they realize that when they are
alone, if they quit biting, they would quit being bitten.  At 3 to 4 months
they are getting their adult teeth, and it seems they spend every waking
moment biting or chewing.  One thing you can do at that stage is to knot and wet a piece of cloth.  Then freeze it.  The cooling will soothe the gums.  Only let the puppy have it when you are there to watch it.  I maintain a Lab's favorite chew toy is another
Lab.  Otherwise they settle for any person they can.  The best technique may be to yelp and walk away, the same a ittermate would if bitten to hard.  

You just have to keep on correcting them, hundreds of times, not dozens.
Provide sturdy, safe toys such as Kongs and Nylabones.  Avoid things they can
chew pieces off and choke on them.  Keep them away from electrical cords.
Crates are essential for most young Labs and other dogs.  

The pet stores are full of toys that many dogs will quickly chew up into
pieces they could choke on or cause intestinal blockages.  If you are not
there to watch, stick to sturdy stuff such as Nylabones and Kongs.  Keep a
close eye on chew toys and quickly discard anything that is coming apart in
pieces.  Rawhide is especially bad because it swells after being swallowed.
These problems are the worst with, but not limited to, large, aggressive
chewers such as Labs.