After my first 6 months of mining, I have come up with a few pearls of wisdom for beginners:

90% of all mining program errors are due to too much overclock/undervolt
The other 10% are caused by errors in configuration
If you’re sure the problem is not the riser, the problem is the riser
Never keep any wallets on a machine you use for mining
It’s hard to mine the most profitable coin because it’s always changing
If you have to sell your mined coins immediately to pay the electric bill, you probably don’t need to be mining (have some working capital)
The pool with the highest hash rate doesn’t make you the most coins over time
Always keep older versions of mining programs, you might need them one day
When a new GPU driver comes out, let somebody else be the guinea pig
Time is money in mining.  If your machine is down, you’re not making money
Fine tuning a rig can be time consuming; don’t let it consume all your time
If it ain’t broke, don’t fix it.

Please feel free to add your own...