Tell Bell - Kelston Products
Check out the website. I know it seems ridiculous at first but Cooper is trained 100%. I bought the Tell Bell and it comes with video and instruction. It took about 4-5 weeks total before he rang the bell and the first time he did I was such a proud mommy!
When I took him out to potty I put his leash on, took his paw and tapped it gently on the bell and went outside. I continuously said "go potty" "coopy go potty" every time until he finally went. I always brought treats outside and made a huge ordeal when he went. If he didn't potty, no big deal. We just tried again after a short while.
I work from home so I had the time to properly train Cooper. When he was just about Charlie's age I would take him outside about every 30 minutes or so. My vet always told me to take him about 5-10 minutes after he ate and also right after he chews on toys.
On another note: I can't get Cooper to do anything else.