As SuperRanger and JW411 have indicated above -
You can exceed, with the airport's permission, on a reasonably regular basis, the allowable PCN by up to 10% without problems. So for a PCN of 50, an ACN of 50+10%=55 would usually be allowed. That could be an extra 15,000 kgs for a Boeing 777. No problems say once a week.
You might be able to exceed, with the airport's permission, on an occasionable basis, the allowable PCN by up to 25% with few problems. So for a PCN of 50, an ACN of 50+25%=62.5 might sometimes be allowed. No problems for something like a one-off series of 3-5 charters. There was a great post by JW411 about dinners on Greek Islands that can help this frequency increase:
http://www.pprune.org/forums/showthr...ighlight=bogie
If you are pushing it, and you exceed, as a once-off, the allowable PCN by 40%, you'll walk away, but there may be some problemxs with ruts in the runway. That is usually no problem for the aircraft, but is for the airport. Hint - the real pros do this at night. If the plane gets off again successfully, the airport doesn't find out until the next morning's runway inspection, by which time you are long gone. I am told that if you feel the aircraft start to bog, give it lots of omph and it usually crawls out, up and away.
How far can you go? Well I reckon the folowing photo was at an overload of 100%, and the 747-400 didn't sink in until it slowed down to 20 knots.
http://www.geocities.com/profemery/a..._practice..jpg
And they all walked away, which counts for a lot.