Please help me simulate worn edges on armor ( some people use drybrush matt black/gunmetal combo or use lead pencil) What do you suggest?

try abit of both and see what you prefere if using paint try using some blacks for scratches ect or some rust colours to show scratches which have been exposed to the elements,
by using pencil you can buff up the lead to give areas of heavy wear like were contact between metal parts or troops climbing in and out of doors ect

take much care to what areas you imply the rust . Some vehicles have aluminium parts for example that never rust. So take care !!! Use 3b soft pencil for warning also

can someone post pics of this technique? I am quite dumbfounded...

For this I used a 50/50 mix of humbrol matt black and blac white, apply with a brush.

You can find more pics in here

I sometimes simulate worn edges by painting the model and when it's still wett sort of smere the paint on the edges so the base coat shows through

I will dry-brush using a PollyS shade of Navy Gray, sorry I don't have the exact name of it here, but it is just enough to bring out the edges.... a little heavier in places that will get the most wear. That is the first color that I dry-brush on... the first paint of the (my) weathering process

I also use artist pencils... silver, browns etc.

All depends on what you are trying to accomplish...