I have a medium size terrier / black lab mix about 6 or 7 years old. He is still very active and is pretty strong. We recently moved to a new house but unfortunately the yard is not totally fenced in which means our dogs have to be tied out. They do not like to potty we are holding their leashes so we tied them out and give them their privacy. They will usually go the second they hear the door close. But our little boy has very quickly learned how to slip out of his harness with in seconds. I have tighten it but He has to be able to breathe so I cannot put it any tighter. We thought maybe if we hooked his collar and harness to the lead outside maybe he might stay put. Well it worked one day and the next he had slipped them both. I tied them out and by the time I made it inside to watch them out the window he was gone. I found him but would like that to be the first and last time he gets away. We have never had any of our 3 dogs get loose before in the 7 years we have had them and it was very scary when he got out. Please any suggestions would be greatly appreciated.

Don't know if this would help
My pitbull was getting loose too so I took a clip type hook and clipped the harness and a nylon collar and a choker chain collar all together now she hasn't been able to free herself any more.

Wow what a little escape artist. Some dogs have learned the trick of holding their breath when you put a harness on to make it looser, so sort of dsistract him or tickle him while you pull the buckle tight. Could he be somehow wanting to go back home to the old house? You may have to walk that one on a lead when he needs to go out while you are finishing fencing in a safe enclosure for him. Just turn your back or walk a few paces ahead when he's ready to "go" and looking the other way say something like, "come on, hurry up, let's go" if he's got performance anxiety. My dog would simply chew through any harness I tried to use.

When the choker collar is hooked to the other two things it really doesn't tend to choke just give extra support.

I had this same problem with my husky. He could get out of any harness in less than 2 seconds no word of a lie. Even an anti-pull harness he could get out of. If you are willing to pay for a solution, than I have the perfect one for you. The escape artist harness. We got it for our dog and it works perfectly, he hasn't gotten out of it since(we had it for about 3 months.)
Now you could try to save money and try to fix something up with what you already have, but are you sure that you would want to take that chance? It only takes one time for a dog to get off and get hit by a car. I also realised this that is why I purchased this item for my dog.

Below is the website where you will find the product. Again it is the Escape Artist Harness. Take my word for it it works great.

Walking Harnesses - Ultra Paws