Yep, a block heater is a very good idea. They are so common that they almost seem like standard factory equipment. Tough to get along without one in cold weather.
Here's a couple of ideas, other than going to an alternative like a lower rad hose re-circulating type heater. First is that I often use 2/3 to 3/4 throttle to start in extreme cold. Second, is that you might want to check the glow plugs. The plugs can be checked with an ohm meter, and I believe most service manuals have specs for plug and wiring resistances. Generally, plugs are either good or bad. They don't get weak, but the connections and grounds can become corroded which slows them down.
Another thing might be to clean the battery and ground cable connections. Poor connections will slow down the glow plugs, but also will spin the starter slower. I cleaned my contacts recently, and the pre-time was halved.
Last idea is to go to #1 diesel fuel. I'd check, but I believe that #1 diesel is OK for most engines. #1 diesel has a lower flash point than #2, so it starts easier in cold weather. It also has a much lower cloud point so it doesn't gel as easily, but it also produces somewhat less power than #2 fuel. Well, guess I'm seldom out of ideas, so a hair dryer near the air intake might do something. However, ether starter fluid is NEVER, NEVER, EVER used in diesel engines.
