Hello Dr. Laura, 
My mother-in-law and I have a wonderful relationship. We built it over many years and continue to do so. I believe it takes two to make it work in any relationship and this one is certainly no exception. My MIL and I both: 
- Try to understand and accept each other, especially where our differences lie 
- Always remember we both ADORE the man who brought us together 
- Never complain to each other about that man who brought us together 
- Look for common ground, like the other people we both love my children / her grandchildren, my father-in-law / her husband, my in-laws / her siblings and other children, and their kids, etc. 
- Take the time to check in with each other once in awhile and ask about the other's well-being, latest pursuits, etc.--in other words, make a concerted effort to maintain the wonderfulness.
But I don't believe it would work if we were not BOTH engaged in these efforts... 
Best wishes, 
Pam