Running out of potty training patience

Sara, I have no idea where to turn anymore when it comes to potty training. I think we have done it all and our patience with our 3-year-old son has run out. I am expecting my second child any day now and my plan was to have my oldest son trained before the arrival of this baby. My sitter is wonderful and has had great success in the potty department with my son. I believe he has been going for her since he was around two. To make life easier for her I started buying Pull-Ups and he never seems to wet them when he is at her house. So to keep the pattern going I asked her what it is she does at her home. She simply calls him and says "Owen go potty". He drops whatever he is doing and goes all by himself. At home we have tried to do the same but it has failed. We have tried bribes (smarties, stickers, etc...), we have taken him shopping to pick out "big boy underwear" recently but he wore them and dirtied them.It seems that if we let him run around without anything on he will go potty by himselfbut as soon as he has anything on, pull-up, diaper or underwear, he will do his potty in that. Is there anything else that we can do?I have sat with him and explained that when wearing his Thomas underwear he cannot pee or pooh on Thomas. I don't know if he actually understands. I am almost ready to give up on potty training altogether sometimes. Any advice you can give would be helpful. We have also noticed that when his friends visit and they go potty with their parents he will go too. Having other kids around seems to make it easier for him.Kellie