When Bentching on Rosh Hodesh Av, do you say Shir HaMa'aloth for Rosh Hodesh or 'Al Naharoth Bavel for the Nine Days?
I once saw a bentcher that said that "our minhag" is never to say Al Naharoth Bavel before bentching except at the Seudah Hamafseket before Tisha B'Av. It makes sense because if you only will have one opportunity to say something, then you will have much greater kavanah for it.
Cf. Before the Mishnah on Brachos 34a on the punishment for someone who thinks that multiple opportunities exist to say Shema in the Zman (if there was a chisaron in kavanah). The punishment is that you hit him with a hammer until he has kavanah!