Be there when she needs you.
Don't take your son's side over hers, unless you absolutely have to.
Don't interfere.
Listen and advise, but don't preach. Instead of telling, use suggestions or questions such as, "have you thought about this or that?"
One thing I do with my sons-n-law (I don't have a DIL) is, when they tell me they are going to do something, I debate with them, making sure that's what they truly want to do. I throw every challenge their way, so they will think major decisions over as best as they can, with as much information as they can get. I give them many angles to think from. They know and understand what I'm up to, for I've told them why I'm debating their plans. If they follow through with their original plans, I know they really wanted to, and had an opportunity to try another way. It's become a process we use, now. Very seldom do they change their mind, and things usually work out. It makes me proud.
And the most valuable thing you can do for your DIL, esp. if she doesn't have a steady relationship with her own mother, is be a mother figure. Even if her mother is a strong character in her life, your "good mother" perspective will only add value to the young girl's life... but, never make it to where she has to choose between the two of you.
AS for your son, never make it to where he has to choose between you and his new wife... I'm sure you knew all of this, but it's what I wanted from my MIL.