I have a stern look too - a "shriveller" as DH and DSs refer to it. Trouble is, it doesn't seem to have much effect on DGCn. In fact DGD, 28mths, now has an even more scary glare of her own that she turns on me when she is thwarted!
My house, my rules, works quite well for me, especially as I almost always look after DGCs at my house, but when I looked after them at their house some six months ago, DGS (then 3.5 yrs) was very quick to say "but that isn't a rule in our house, Grandma" when I asked them to do something that didn't meet with their approval. Don't you just love 'em 
My biggest 'discipline' tool is a long-term one - no matter how much screaming (or other behaviour designed to wear me down so that they can have their way) they do, I will not be swayed once I have issued a firm instruction. As a result, that kind of behaviour is now rare when I'm in charge. They save it all for when their dad, and particularly their mum, are in charge.
That said, I save those firm instructions for when it matters (and when I am in a position to see it through) - most of the time I am very flexible and happy to let them have a say in what we do and when we do it. The last thing I want is for them to grow up thinking of me as the wicked, authoritarian grandma! That would be awful 