Tell how beautiful she is, give her a day out while you stay home and cook, clean, laundry and watch the other children. That will go a long way for her to keep the spark alive. Right now she's too exhausted!
Date nights weekly. Heck, you can even take the kids to a drive inn and let them sit in the front when you can't find a sitter. No excuses. But DO GET OUT ALONE MOSTLY...and even with friends once in a while on top of this. Go for walks