What worked for me was taking my Basenji for a short walk along the road that I live on every hour until she went. After just a couple of days it made a big difference and after that I increased it to 2 hourly, 3 hourly and so on. I think it worked because she became used to going outside and it felt less familiar going inside. Going outside and waiting for her to perform with her getting stressed, wanting to go back inside and not understanding what I was asking didn't get us anywhere and this was much quicker and long lasting. Now she goes for 2 decent walks per day (morning and night) and usually asks to go into the garden once in between (she is 18 months old). My Basenji was a puppy so if you decide to try this you may not want to start at hourly. The other benefit of this is that she doesn't tend to want to poo in the garden unless she has an upset stomach.