Know when to zip your lip
Tact is a highly desirable quality in a grandparent. Try not to give advice unless you are asked for it, and even then, tread warily. Young parents are sensitive to any hint of criticism of their methods of child care.
However much we may disapprove of a toddler being offered a choice at mealtimes, as if in a restaurant, or a four-year-old sleeping in his parents' bed every night, we will, if we are wise, keep quiet about it.
Never, ever say ...
A maddened mum begged me to pass on this message: 'please, please, please don't tell us that we were all dry through the night by 18 months. No matter how many times I hear this I DON'T BELIEVE YOU!!!!'
When looking after babies or toddlers, wear clothes with large pockets and fill them with tissues. You'll need them to wipe noses (there is virtually no closed season for runny noses), chocolate-y mouths, sticky fingers and grazed knees.
Keep, in your handbag, your car and your house a supply of treats to hand out as rewards, bribes and comforters. If sweets are taboo, make do with raisins or organic fruity snack-bars.
Make your house safe
Don't be the granny whose grandchild locked himself in the lavatory, fell out of the window, or drank the washing-up liquid.
Check your house for safety before grandchildren visit, putting ornaments and pot plants out of reach (out of the reach, that is, of a child standing on a chair).
If floor level cupboards don't have locks, or you have lost the key, tie the handles together or seal temporarily with masking tape.
Make yourself popular with the parents
The one thing parents of little ones long for above everything is a lie-in in the morning. To make sure they get it, invite the grandchildren into your bed for a morning cuddle and story, give them breakfast and get them dressed.
Make yourself popular with the children
Keep a few toys and books at your house for each age group, preferably different from those they have at home, so that 'Granny's toys' and books seem special.
They don't have to be new and expensive - charity shops and car boot sales are good sources, and you can have fun seeking out vintage favourites from your own childhood.
Practice makes perfect
We grandparents all admit to having trouble folding and unfolding the buggy or pushchair. Don't get caught in Sainsbury's car park struggling with a bundle of tangled metal while the baby screams blue murder in the car and the toddler disappears over the horizon. Practice, practice and practice till you have mastered the art.
Acquire the knowledge
Learn the difference between Tinky-winky, Dipsy, Lala and Po. They are today's equivalent of Flopsy, Mopsy, Cottontail and Peter, or Florence, Zebedee, Dougal and Brian.
And be reassured that Thomas, Edward, Henry and Percy still occupy adjacent engine sheds, still under the watchful eye of the Fat Controller, and as popular as ever.
It's okay to say no
When a request for child-minding comes along, many grandparents will drop everything, even at short notice. But don't feel obliged to do so. We have lives of our own to lead, and our children must understand that.
One mum explained, 'Crucially, my mother-in-law sometimes refuses, so, when she agrees, I know it's all right and I'm not imposing on her.'
Keep in touch
If you don't see your grandchildren regularly, keep in touch by telephone, email or texting and they are less likely to be shy when you meet.
Even babies, who can only gurgle in reply, like the sound of a familiar voice on the phone. Older children also love getting postcards and occasional little presents through the post.
Be the family historian
Do tell your grandchildren all you know about family history, and what life was like when you were young. It may make them yawn now, but if you don't tell them about the characters in the family album, when they are older they will wish they had asked you whilst you were still there to satisfy their curiosity.
Your second childhood
All too soon they will grow up, so enjoy their company while you can. Indulge the child in yourself, join in their games, whether it involves crawling around on the floor being a dinosaur or putting on a tinsel crown and being a princess.
Jane Fearnley-Whittingstall is the author of The Good Granny Guide and The Good Granny Cookbook.