1. Your child is not perfect; don't talk as if they are. You raised this child, you sacrificed to bring them into this world and to guide them through it. Now someone else has taken over as the number one person in their life. It may be hard to imagine anyone being good enough for your child, but you need to remember that their mom is thinking the same thing and probably neither of you are right. If you talk to your son or daughter-in-law as if your child is perfect, it just may make them focus even more on the things that are not perfect. Not to mention that they won't feel comfortable around you because they'll feel like you're only seeing their imperfections.
2. Don't take sides. As hard as it can be, as wise as you may have grown through years of being married and seeing how life works, don't side with either of them when they're not getting along. No matter what, when they make up the one you didn't side with is going to remember. You can't win in this situation. Plus, when they make up, the one you did side with may not be grateful anyway, now they'll just remember that you thought their beloved spouse was wrong.
3. Don't offer marital or relationship advice. Even when it is freely asked for, keep it to a minimum and keep it neutral. Don't compare their situation to one you went through because then if they don't make the choice you're recommending or that you made, they're silently implying that you were wrong. That makes them feel bad and may leave you feeling hurt or offended. They're going to find advice all over the place, and odds are they'll ignore it all and figure out life on their own, but if they don't like your advice then they may avoid you just to avoid admitting that they didn't take your advice.

Just remember, it can take years to build a great relationship of trust. They're starting on the hard process of building that between themselves; any bumps that you throw in can be overcome, but will be remembered and can lengthen the time it takes for your relationship to grow. You don't want to be an evil mother-in-law so don't be a counselor, advisor, or therapist, just be a neutral friend, a shoulder they can lean on and ear they can talk to that doesn't respond with more than love and comfort.

Being a mother in law should be a pleasure and a joy. You do not want to risk alienating your new son or daughter in law because you are too nosy, too rude, too controlling etc. You need to be sure that you express opinions as opinions without the expectation that they will be followed. If you act like you want to control their lives, even though you are no longer financially or legally responsible for your child, you will be labeled the evil mother in law. Instead, enjoy them. Enjoy your grand kids. Enjoy the fact that your child has grown up and moved out, even if they are making some poor choices. Do not meddle. Meddling equals trouble, trouble equally evil mother in law. Don't do it!