First of all:
Make sure you spend equal amounts of time with EACH DOG so that no one dog is excluded and gets sad or jealous.
Second:
Don't treat one dog this way and another that. E.G.: You put your old dog in a cage at night regularly (or as a punishment if its not normal) and then let your new dog wander free. Your old dog will hate the new one even more, because he/she thinks to liek the new dog more than him/her.
Third:
If they fight over objects, give them each an object that is the same thing.
Fourth:
Remember, dogs naturally have this idea of pack territory. Maybe your old dog feels this new dog is a threat to the territory. Just show that he is a friend by allowing him to do things you allow your old dogs to do since they are yours, then your old dog will eventually realize that he isn't a threat. I had this problem with two cats of mine, but I just made it clear they were equal, and I was "alpha".
