Tips for marking places.
1)Never mark an instance IF you plan on returning the next or + days
You'll soon notice, that oh look its the next day. Oops, it looks like the instance/mark has expired and wont let me return
2)Mark popular places you wish to return to often
If you plan on farming in a certain area, this is very helpful. Just make sure its not an instance. Or if you want to mark the pet pavillion- because you dont want to go ALL the way back from Ravenwood, :p.
3)Oh no! My mana is almost gone, but I want to farm some more!
That little voice might say, keep farming! Dont listen to it, instead of running out of mana and having to return to this place by getting back by walking. Use that mana on marking it so you can return after its been marked.