I think generally the accented syllable in English is elongated. For example, in "chocolate" the "ate" part is accented, so you get チョコレート. If you know English pronunciation well and the 外来語's source language is English, you can probably guess pretty well how to spell words in katakana. Of course, there is still the matter of checking them to be sure.
Now that I think about it, I'm pretty sure there are quite a few rules/guidelines about spelling English words in katakana in the Dictionary of Basic Japanese Grammar (or was it the intermediate version?) that also had to do with what sounds that exist in English but not in Japanese (like both "th"s) correlate to what sounds in Japanese. You may want to check that out if you can.