I live in Los Angeles, so there are usually native speakers of every language around for practice. For Japanese (which I learned a decade ago back in college), I watched a lot of anime, dated Japanese girls, and ate at Japanese restaurants before spending a few months in Japan. For Italian, a have a few Italian friends so I tortured them with my terrible accent for practice, and listed to opera. French was easy because both my parents speak it, so I practiced on them, plus watched a lot of French movies. Mandarin I practice with my coworker (although as a Cantonese Malaysian, Mandarin is not his native dialect so he has an accent just like me), and whenever I go to a Chinese restaurant. My latest language is Brazilian Portuguese, which I am learning in anticipation of a trip to Brazil. I'm also learning Capoeira and learning to cook Brazilian BBQ, so I practice a bit at my local churrascaria.

In addition to these languages, I also learned Spanish starting in pre-school, and I speak a little Tagalog (my wife is from the Philippines). In answer to your question about getting the languages mixed up, not really. I learned them at separate times, so I think they're stored in different places in my brain.