Most people just have a single PPP connection to the Internet, and don't want anyone coming back into their network, or the firewall:
## Insert connection-tracking modules (not needed if built into kernel).
# insmod ip_conntrack
# insmod ip_conntrack_ftp
## Create chain which blocks new connections, except if coming from inside.
# iptables -N block
# iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
# iptables -A block -j DROP
## Jump to that chain from INPUT and FORWARD chains.
# iptables -A INPUT -j block
# iptables -A FORWARD -j block