As you said, you'll just have to memorize these... Sometimes, gender corresponds with the physical gender ("en gutt"), but a lot of words are just about knowing it. As for 'et jordbær': all berries will be neuter, since 'bær' is neuter. So it's "et blåbær" (a blueberry), "et kirsebær" (a cherry), "et bringebær" (a raspberry). And so on!