A little baking soda mixed with vinegar can help. Be careful as it can be pretty "fizzy". Shouldn't hurt your pipes, though.
I hear this is good once it's clean and you have built up crap from over the years, but not so sure it will unclog what you need. I was told to do the baking soda/vinegar mix about every couple of weeks, then cut back to once a month. I'd Draino it in a heartbeat. Gel it's arse pipe.