You'll want to pack the brown sugar down fairly tightly into the measuring cup. Tight enough so when you pour it out into the other ingredients it retains the shape of the measuring cup.
Packing is definitely the way to measure brown sugar accurately. I can't think of any recipe I have that does not specifically instruct me to pack the sugar into a measuring cup.
Definitely pack it in!!! If you pack it tight enough, it should often slide out as a mound still stuck together (especially if you live somewhere with some humidity) I use a measuring cup and pack it all the way to the upper lip and then smooth it out.
I measure my ingredients in ounces on my kitchen scale. One cup of brown sugar is 8 ounces. No need for all the measuring cups and not all measuring cups are equal.