My milk supply diminished slowly and then was gone by about month 7. From that point on with my DS I coudn't let him nurse anymore because it really hurt a lot. He was easily distracted during the day with food, other beverages, other activities, hugs, etc. He still really wanted to nurse for naps and bedtime though. For those times I started counting to 10 and then I would have him turn and face the other way and rub his back. Sometimes he would turn back around and I would let him cuddle. In his own words, "I just cuddle the boob". Usually after a few minutes I'd have to turn him around again because he would want to nurse again. Then I'd rub his back or legs, etc, until he fell asleep.
Now DD is 3 weeks old and at first I did let DS nurse a bit. For the first week or two I gave tandeming a try...and it didn't work for us. We are working on day 3 now of no mommy milk after having some for a few weeks and he really seems okay with it and so am I. He does ask for mommy milk at times but he hasn't given major protests and has been fine with other food and drink and cuddles and hugs and kisses, etc. I tell him his sister doesn't have teeth so she can only have mommy milk, that he has teeth and can eat food. It's cute, when his sister cries he says, "Baby needs mommy milk".