Some tricks learned during MDMA beta testing, from:

Using the modem's network interface (NDIS) while connected to a LAN
By default, Windows assigned a metric of 10 to my LAN adapter and 25 to the modem's network interface. This means that internet traffic preferred to go via my LAN adapter unless I unplugged it.

To access the internet via the modem's network interface and still access LAN resources via the LAN adapter, simply change the metric of the LAN adapter to 50 in the advanced TCP/IP properties of the adapter.

Below is the output of 'route print' after changing the setting.
Note is the router on my LAN and is the IP address assigned to my modem by Cell-C.
>route print
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...e0 cb 4e c8 98 ea ...... NVIDIA nForce Networking Controller - Packet Scheduler Miniport
0x10004 ...0c 5b 8f 27 9a 64 ...... HUAWEI Mobile Connect - Network Card - Packet Scheduler Miniport
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
       50       25       25       25       1       50       50       50       25       50       1       1
Default Gateway:
Persistent Routes:
Connected but no throughput (NDIS connection)
If you are connected using the modem's network interface but don't seem to getting any throughput, make sure that the TCP/IP properties for the adapter are configured to 'obtain an IP address automatically' and 'obtain DNS server address automatically'.
You can check by typing 'IPCONFIG /ALL' at the command prompt and making sure the adapter does not have a 169.254.x.x address and that the DNS servers' IP addresses are sane. This seems to be an issue with Windows 8. (Thanks morkhans)

Unbind unnecessary services from the NDIS interface
By default, Windows binds Client for Microsoft Networks and File and Printer Sharing for Microsoft Networks to all network interfaces. You do not not want these on your public interface. Unbind them and probably everything else except for TCP/IPv4.