What are some methods for conserving IP addresses for serial lines?

VLSM and unnumbered point to point interfaces are the obvious ways.

The 'ip unnumbered' subcommand indicates another interface or sub-interface
whose address is used as the IP source address on messages that the router
originates on the unnumbered interface, such as telnet or routing messages.
By doing this, the router is reachable for management purposes (via the
address of the one numbered interface) but consumes no IP addresses at all
for its unnumbered links.

When a serial ip interface connects several sites, as an SMDS link might,
then the use of an appropriate subnet mask (and a routing protocol that can
make good use of the information) will minimize address consumption.