I am not a dog trainer, but I've worked with several on my dog and I've learned a bit in the process.

Do you have a regular walking schedule--like at least 30-45 minutes in the morning and in the evening? Getting dogs, particularly smart ones, on a walking schedule seems to help with a host of issues. Also, being such a large dog, Sara probably needs lots of exercise (i.e., walking twice a day, and not just around the block). Like others have mentioned, some dogs tend to be more picky about where they do their business than others. Mine will not go in my yard (or the house)--he has to be on a walk, usually about 30 minutes into the walk. He's been very well trained that walks are the time for business. I got him as a 2-1/2 year old rescue dog, so I didn't train him to do that, someone else did. With such a small yard now--so different to what she's grown accustomed to--she might really benefit from two long walks a day, no matter the weather. This teaches them that leashes are not only for fun trips to the dog park, but for going on walks, which is their job. Dogs need jobs.

Speaking of dog parks, several dog trainers I know really do not think they are a good idea because all rules go flying into the wind, in addition to the health issues that can come up. My dog does not play well with others, so I don't have to make a personal decision on the dog park issue (we don't go for other reasons), but it's something to think about and maybe ask around about.

Also, feeding times are really important. I have a friend who has a dog who cannot seem to hold it during the night, so she feeds him only once a day, in the morning. It works great and he no longer poops in the house at night.

Put up a bell on the door so Sara can learn to ring the bell when she needs to go out. Make sure to knot the string a few times so she can't get it stuck around her neck (happened to my dog once and it was not very much fun).

But mostly, just go on two long walks a day and I bet she'll get the hang of it.