Don't be concerned if you're not sure which is right. Usage in this respect is quite mixed. Why should this be, though?
In the western US I was told that the divide on this usage has to do with the difference in the way the word "Historic" is pronounced.
Where the British would pronounce the word as "istorik" the usage would follow the convention of using "an" before the vowel sound.
Where us yanks would pronounce the word "historik" the usage would be follow the convention of using "a" before a consonant sound.
Have you heard this, or is this a good rule to employ?
I agree with Bud's explanation. Pronunciation is probably the origin of the variation in usage. Some British accents involve dropping an initial "h"; thus, "historic" is pronounced "'istoric" by some speakers.
I don't think we can say, though, that all British people drop the initial "h" or that all Americans don't. Usage is more varied than that.
Few speakers seem to know the reason for the two usages. Many thus say "an historic" simply because they hear others around them saying it, not as a result of any accent. This may be the reason that the usage has spread.
A quick bit of Googling reveals that the phrase "a historic" is used on the web 73% of the time, with "an historic" used 27% of the time. Of course, the web is a written medium, not a spoken one. These numbers might not reflect their usage in spoken English.
In summary, the form you use seems to be little more than a personal preference. Both usages are sufficiently common to be considered correct in contemporary English.
Hope this helps.
