For example you've added a static route:

route add -net 10.0.0.0/24 192.168.10.1

You want to make it permanent after reboot, in that case edit /etc/rc.conf and add the following line:

static_routes="staticroute"
route_staticroute="-net 10.0.0.0/24 192.168.10.1"

If you want to add multiple routes you must put them in static_routes= variable like in the following example

static_routes="static1 static2"
route_static1="-net 10.0.1.0/24 192.168.10.1"
route_static2="-net 10.0.2.0/24 192.168.20.1"