openvpn --genkey --secret static.key

Then copy this key to both client/server.
server-static.conf:

  # run with: 'openvpn --config server-static.conf'
  mode p2p  # default
  proto udp # default
  dev tun   # default
  port 1194 # default - server will listen on all interfaces, on this port
  dev tun
  ifconfig 10.4.0.1 10.4.0.2 # first Tunnel-IP is local, 2nd Tunnel-IP is remote
  verb 3
  secret static.key # openvpn --genkey --secret static.key
  keepalive 10 60
  persist-tun
  persist-key
  persist-local-ip
  comp-lzo

client-static.conf:

  # run with: 'openvpn --config client-static.conf'
  mode p2p  # default
  proto udp # default
  dev tun   # default
  remote [server ip] 1194 # client will connect to this (server) IP + port
  ifconfig 10.4.0.2 10.4.0.1 # first Tunnel-IP is local, 2nd Tunnel-IP is remote
  verb 3
  secret static.key
  keepalive 10 60
  comp-lzo
  explicit-exit-notify 2


firewall and port forwarding
firewall.sh:
  
  # run as 'sudo sh ./firewall.sh'
  iptables -F                                                                          
  iptables -X
  iptables -t nat -F
  iptables -t nat -X
  iptables -P INPUT ACCEPT
  iptables -P FORWARD ACCEPT
  iptables -P OUTPUT ACCEPT
  
  SERVER='paste IP address of the server here'
  
  iptables -A POSTROUTING -t nat --match iprange --src-range 10.4.0.2 -j SNAT --to-source $SERVER
  ## low ports first
  iptables -A PREROUTING -t nat --dst $SERVER -p tcp --dport 23:1024 -j DNAT --to 10.4.0.2
  ## other ports (watch not to forward OpenVPN port 1194!) 
  # iptables -A PREROUTING -t nat --dst [server ip] --dport 1200:20000 -j DNAT --to 10.4.0.2
  #
  # echo 1 > /proc/sys/net/ipv4/ip_forward

complete and (way) more secure setup: http://john.de-graaff.net/wiki/doku.php/links/openvpn#server_tls_mode_with_client-auth 

automated setup (nice!): https://github.com/Nyr/openvpn-install