Things I have learned or learned through other's mistakes

when watering/feeding use about 1 gallon of water per gallon of soil. Using more water reduces the chances of yukky build-ups in your soil causing ph problems and lock outs.
The use of a dehumidifier and A/C especially in a smaller growing environment is key.
slowly increase your nutes and DO NOT use ANY until you have grown about 4-6 sets of leaves on your seedling.
Slacking on your PHing will cause more time in damage control than it would have to make sure it was right in the first place.
Humidity levels during flowering need to be as close to 50% as possible. Otherwise you will have un-smokable moldy weed
Temps in your grow room over 85-up (should be about 70-75 degrees with lights on) are killing your plants.
If in a small grow space (small closet) if you spend a lot of time in there, they are probably getting enough CO2 from you breathing on them. ( I spend a lot of time with them)
Dry ice is another good source of CO2 in a small closet grow

MOST IMPORTANTLY:

LISTEN TO YOUR PLANTS! Really pay attention to them. They will tell you what they need. If you wait until the problems are way out of hand, you caused more damage to them in the long run and it will cause you a lot of headache in the end.
Good luck and happy growing.