How much is too much?!

I like my friends...
As people who bring smiles to my life, i value them and hence feel like doing things for them that would make them feel good and comfi. However at times, i have been forced to examine my over-enthusiasm that has left a bitter taste on the taste buds of my mind, making me wonder who is going wrong and how much...is too much?!
There are times when you come across these buddies whom u help with all the good intentions in your noble heart! but soon the help backfires and you find your hand that extended help is now suffering from aching fingers!!What do u do then? Do you put your foot down and take back the help offered bcoz the other party has started taking things for granted or you continue suffering and pay for your over enthusiasm!
Talking, is a solution but that again has stages- polite, diplomatic, hinting and then 'To the point!'"To the point" is the last resort and u stand the risk of making things change, more often for the worse and hence the question is when do u take this last resort...when do u conclude that with all due respect to your beloved friendship and emotions- you have been taking too much of flak?
The simple solution seems like putting an end to that, which is causing harm but sometimes things are not that simple...or are they?