Bradley Schaefer (now with LSU) did a lot of pioneering work with the limits of telescopic vision. Calculators based on his work appear at:
http://www.go.ednet.ns.ca/~larry/astro/maglimit.html
and
http://www.ilangainc.com/limitingmagnitude/
and the article is here:
http://web.telia.com/~u41105032/visual/Schaefer.htm
Basically, scopes go a lot deeper than the old books and charts tell us. I was regularly getting to magnitude 15.1 to 15.3 with my 8" (using Roger Clark's test clusters), and even reached 15.6 on an exceptionally dark night. These calculators tell me that that was to be expected, and that the performance I saw was only average.
The calculators allow you to enter in light pollution info and take your experience into account.
As for maximizing the potential of your eye:
--don't smoke
--don't drink for at least 24 hrs before viewing
--allow 45 minutes to dark adapt
--maintain blood sugar levels by eating during the night.
--make sure to get enough (but not too much!) Vitamin A in the diet
--be well-rested (don't exercize on the afternoon before observing)
--stay warm
--never use any color other than red light, and no light at all if you can.
--don't use a laptop computer at the scope
--surround yourself with dark or black surroundings and use a black cloth on the ground if necessary
--if the sky is not exceptionally black, don't look up at the sky. If you have to to find the object, look down for a couple minutes before viewing the details in the object.
--use a black drop cloth over the head to block out peripheral light at the eyepiece (difficult to do without fogging up the eyepiece)
Don