Live as close to their mother as practical. Especially as they enter their teen years, their peers are more important to them than their family, and having to maintain two different sets of friends because your two houses are far apart sucks. (Of course, I’m basing that off my own experiences; maybe it’s different now with texting and Facebook.) Try not to take it too personally when, as will inevitably occur, one or both of them doesn’t really want to come spend the weekend with you because their friends are all going to the beach. It’s not you, it’s not the divorce, it’s just how teens are. If they lived with you full time, they’d be doing the same thing, and it’s entirely developmentally appropriate. Find a way to be flexible with your schedules so that visitation works for them, as well as you.
Consider referring to your homes without using the phrases “Dad’s house” and “Mom’s house”. I always kind of wondered…if that’s Mom’s house and that’s Dad’s house…where’s MY house? We (my ex and I) refer to our abodes by the street name, instead: “Is your book at Main Street or is it at Fifth Avenue?”
Everyone tells you not to “bash” or “badmouth” your ex, and they are absolutely correct. However, your kids are old enough to understand that everyone has strengths and weaknesses, and if you pretend to always be delighted with your ex, it’s going to cause some ugly cognitive dissonance. It’s okay, within reason, to express frustration or disappointment with her actions…just try not to harsh on her as a person. “Mom forgot the permission slip again? sigh Okay, print off another copy and I’ll sign it.” is okay. “That stupid bitch, she’s always been an irresponsible…” is not.
Let their teachers know what’s going on. They may - consciously or unconsciously - act out in school as they process all this change, and it’s only fair and kind to give the teacher a heads up. If s/he knows, s/he can more effectively help them through this.
Mostly, when the ex is really driving you crazy (and she will, sooner or later) take a deep breath and remember that you once loved this woman enough to create life with her - twice. She must have some good qualities, so try remember them. The kids are the important ones now, and you *can *be a good coparent with an ex. My life is my cite - my parents were very good with me, and my role models for my successful divorce and coparenting with my ex.