Wear short, easily driven fins and use a relatively rapid but gentle flutter kick. Don't get long-bladed or overly stiff fins, as these don't work as well on the surface. Make sure to pace yourself. Try to keep your fins from breaking the surface if possible.
I doubt if they would allow this, but a pair of hand webs will make you go like a rocket once you've built up the strength and stamina to use them properly. If you reach this level then you will also have no trouble with the 400m free swim.
Here's an article that might help with your fin selection. However, keep in mind that all tests were done underwater and the results don't accurately apply to surface swimming:
http://www.scubadiving.com/article/18_New_Fins/
Also, as pickens 46929 mentioned, your snorkel can end up being the "bottleneck" if it restricts your airflow too much. Try this with your existing snorkel: Can you inhale deeply and rapidly without feeling that you are sucking air through a crevice? Can you exhale forcefully without feeling excessive back pressure? (Cover the purge valve for this test). Most snorkels will fail this test, since they are generally designed for easy cruising, not power swimming. All of those gadgets they stick on them (splash guards, dry valves, etc.) tend to muck up the airflow, although some are worse than others. A simple, large-diameter J-tube without any valves or splash guards will provide the least airflow restriction, but make sure you are able to keep it clear without struggling or you will defeat the whole purpose. If you want some splash protection and a purge valve, the US Divers Aqua Lung Impulse 3 is a good-breathing conventional model, as their unique anti-splash design hardly restricts the airflow at all. There are also the various front-mounted snorkels that are used by certain speed swimmers during training. I've never tried one, but they are obviously designed to provide excellent airflow.