Ok, occasionally I have weighed in here in connection to matters on which I am less qualified than many of you. But on this subject I can speak with confidence.
I would disagree with those who say that any dark rum will do. Myers, for example, makes a horrible D&S. Goslings is the default choice and for good reason - it just goes particularly well with ginger beer. An excellent and higher-end alternative is Mount Gay XO. It adds an oaky flavour that is really quite satisfying.
As for the correct preparation: The practice of pouring the ginger beer first and the rum on top is a matter of presentation only. The dark rum floating like a storm cloud on top of the comparatively lighter coloured ginger beer speaks to the name of the drink. Ultimately as others have said, mix it up to enjoy the way it was meant to taste - or take a good pull off the rum first if it's been a tough day on the water.
There is no right answer for the amount of rum, other than to say it should be no less than 1:3 with the ginger beer. 1:2 is excellent. You should drink it wth lots of ice which will cut the rum anyway. A weak D&S is just kind of a cloying sweet drink (which may be Gunni's problem).
There is some question about the amount of lime to add. Many cocktail bar versions use quite a bit more lime than necessary which changes the character of the drink. A good squeeze from a decent sized lime wedge (which remains in the drink) is all that is required.
One of the things that makes this drink truly fun is the selection of ginger beer. There are so many out there and most are regional so extensive experimentation is warranted, both at home and when traveling. I tend to look for a strongly spicy variety and try to avoid those that are sweetened with high-fructose corn syrup. (That stuff will kill you). One rather high-end suggestion is "Fever Tree". Check out:
http://www.fever-tree.com/ginger-beer
Lastly is the issue of snacks to accompany. I would never normally eat cheezies (Americans call them cheese doodles?) but in Canada we have a brand called Hawkins that make a perfect accompaniment for the D&S. Hey, nobody's perfect.
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