Probably the first step in securing snmp access is to create good community strings (and control who has the community strings). Also you can configure access lists to work in conjunction with the community strings to limit who has snmp access to the router.
I would suggest the usage of SNMP v3 otherwise it wont matter how strong you create your community strings because they will be sent as clear text, and someone with a sniffer can eassily decode them. Here is a link on how to configure SNMP v3
