Remember - If you photograph somebody in the street, you should be asking permission to do so, its basic manners in my eyes. Just ask yourself "how would I feel if someone stuck a camera in my face without asking"? - Of course, situations don't always provide this opportunity, language barriers etc can cause issues, but as a general rule, I ask 99% of the time before I even get my camera out of the bag.
Something I like to do if possible is to return to the person as soon as I can after a shoot and offer them some prints of themselves, I usually explain this when I first start 'breaking the ice' if I know I will have the opportunity to return to that location at a later date. Again, bear in mind that if someone asked to take your photo and offered to either email it to you or give you prints for free, I'm sure you would be far more willing to participate than if there was nothing offered - its just human nature...
In some countries, people just want a 1USD bill to make their day, I personally have no problem with this and have used 'Benjamin Franklin' many times to help gain access into villages, houses, factories and many other facilities where normally there might be little or no chance to enter. It's a simple fact of life that in some places, 'blagging and bantering' your way in might not cut it, so cold hard cash could be the only way you are going to get the shots you want.
Today I returned to Lamma Island in Hong Kong where I shot some video and stills with Mikey from www.lightenupandshoot.com early last week. I wanted to return and visit the dear old lady who posed for me towards the end of the day and give her some prints. She offered to take me to her house for dinner afterwards (again - she offered this the first time we met her also). She was very grateful that I had fulfilled my promise to bring her some photos - it made both our days!
This is why I love being a photographer, seeing people smile ear to ear when they see the images you have created is a feeling I'm sure most jobs cannot provide - I'm very lucky to be able to do this full time, and I look forward to waking up tomorrow and heading out to find my next subject...