Are there conventions for representing hierarchical components on a schematic?
Sadly there is no accepted standard. As with most schematics, whatever works to convey the information best for your group is all that matters.

Each tool is also going to implement hierarchical designs in different ways. Altium for example, allows you to add suffixes to reference designators. One way for you to do it would be to have R1_1, R1_2, R1_3 .. U1_1 U1_2 U1_3.

http://techdocs.altium.com/display/ADOH/Multi-Channel+Design+Concepts

It's a handy feature in theory but sacrifices the ability to set reference designators based on PCB placement which is much handier when you need to change out R1_2 but now have no idea where it is without searching your layout.