Portuguese vs. Spanish: How do you stop mixing them up when learning both simultaneously?

I've been learning both Portuguese (Brazilian) and Spanish for about six months now, and I'm constantly getting words and even grammatical structures mixed up between the two. One moment I'm speaking fairly well in one, the next I'm accidentally throwing in phrases from the other. It's becoming a real roadblock to progress. Has anyone successfully managed to keep them distinct in their head, and if so, what strategies worked for you? I'm desperate for some practical advice!