Our discussion of the problems of deciding how to pluralize Latin and Greek nouns in English (here, here, and here ) leads me to point out that those who have difficulty with this can take some comfort from the fact that the Romans themselves did not always follow the Greek accurately. Steve of Language Hat referred me to a recent discussion of classical plurals in which Justin points out that octopi is not quite as ignorant as it sounds since the Romans themselves sometimes treated Greek words ending in πους [pus] as second declension. He gives the example of the related noun polypus, which should be third declension but occurs with second declension case forms. Interestingly, one of the citations in this Perseus entry is to a line of Plautus in which we find the accusative singular polypum, a second declension form (the third declension form would be polypoda). Plautus was very familiar with Greek - indeed, his plays contain many pasages in Greek - so he was surely not ignorant of the Greek form. That even he would shift the declension shows that this must have been a common phenomenon.
Gildersleeve and Lodge's Latin Grammar has a discussion of the declension of Greek nouns at pp. 32-33. They say that many Greek nouns have mixed declension in Latin, with second declension forms used alongside third declension forms, as with polypus. However, they also say that this mixture is pretty much restricted to the singular; the Romans usually followed the Greek in the plural.
Gildersleeve and Lodge also point out that the Romans sometimes took the accusative of the Greek word to be the stem. For instance, Greek κρατήρ "punchbowl" is a third declension consonant stem noun with accusative singular κρατῆρα. It shows up in Latin both as a third declension masculine, nominative singular crater, genitive singular crateris, as it ought to if one follows the Greek closely, and as a first declension feminine, with nominative singular cratera, genitive singular craterae.
By the way, there's a handy summary of the basics of Latin plurals here.