Some good suggestions above. Perrone's spot on with the idea that Thinsulate loses effectiveness over time, so your rental undergarment may well not have been nearly as effective as it should have been.
A few more to offer, from someone who's spent three years trying to figure out how to maximize bottom time in cold water: A really good hood makes a huge difference. It should fit snugly and not permit much water circulation. A lot of people locally use the 12 mm Otter Bay hoods -- That will probably be one of my next gear purchases.
Second: Be careful about getting cold BEFORE the dive. I often drive to the dive site in my undergarment, so that the time spent assembling gear is not time spent losing precious body heat that I could have used underwater. I view heat as a consumable resource very much like breathing gas, and I don't like to waste it on the surface.
Third: NO HANGING OUT ON THE SURFACE! Dive plan, gear checks, etc. should be done on land if the water is that cold, unless the air temperature is so high you can't stand to be in it (in which case the water temp is rarely that low). Time spent waiting for buddies to get in the water, or fiddling with gear, is heat spent that didn't buy you bottom time.
Fourth: In very cold water, SWIM. I have learned that it takes only a little activity to stay much warmer, but dives like drift dives, where I'm doing nothing, result in getting cold very fast.
Fifth: Consider Argon. I know the only study published on it didn't support its use for divers not using helium, but my personal experience is that it makes a difference. My husband agrees with me.
Sixth: Depending on your level of experience in a dry suit, consider running the suit a little looser. For a long time, when I was new, I inflated the suit only enough to take off the squeeze, and used my wing for buoyancy. As I've gotten more skilled, I've added more gas to the suit, and it does result in staying warmer. The downside is a more unstable bubble, and you have GOT to know how to vent and anticipate buoyancy changes or running the suit loose can be downright dangerous.
I would also second the suggestion for optimizing the insulation on your feet and hands. Dry gloves are a must for me.