Ok, what is the number one lesson you see all the time at any book/video/tutorial, for playing 6max NL ring games?

NO LIMPING!

I've read it a thousand times. You get to extract more value on your made hands, you clear the field of players, disguise your hand better, blah blah...
So, why do I have a VPIP 4x larger than my Pre-flop raise? Beats me.

I don't have much time at the moment to post the hard data, but if you take my word on it, this are the numbers I have for my 10K sample:
- I'd have nearly 50% more profit if I'd played only my raised Pre-flop cards (33% of that profit) and had not limped any hands but Pocket Pairs (the other 17%). Of course this would mean either I'd had to tighten my game A LOT (I'd nearly chop my hand range in half), or I'd have to raise a lot more hands (with an unpredictable outcome).

Now, from my numbers, it's still profitable to limp TT- Pocket Pairs while set mining. This would perhaps make my hand range somewhat predictable but, at 2NL most people just don't observe your play anyway. It's probably something I'll have to change in the future but for now I'm still open minded on this one...

Either way, with the exception above for low pairs, NO MORE LIMPING for the next 10K hands. Period.