The most important thing to remember is that everything in life is about sacrifice. Last semester I had two independent research projects in two different discaplines, outreach and public service projects, and 11hours of coursework, as well as EMT-B class. At the end of the semester, I passed my EMT-B exam, learned a lot in one research project and did enough good work in the other to start thinking about writing up the manuscript, and was successful in all of my extracurriculars. However, I earned a B- and a B+ in two of three important science classes that semester.
To me, the coursework wasn't nearly as important as the "real life" experience, so I made those sacrifices. This semester, I'll be working my ass off in my science classes to compensate, but I'm also taking fewer hours. What is important to remember is to always be introspective and observant, regarding your situation and your capabilities. Try out a multitasking strategy for one week and if it works, stick with it. If it doesn't try something soon ASAP. People are often capable of a lot more than they do, because they're 1) apathetic or 2) inefficient. Find your motivation, foremost -- then watch yourself and try to find the best ways to fit the pieces of your life together. This may mean studying for the MCAT during the downtime of a Western Blot, or simply reviewing class notes after each lecture to make sure you understand important concepts (this is important for effective studying).
In sum--- prioritize and sacrifice. Good Luck =)