I've been going back or forth on it, but I've generally been sticking to Russian and German, with a few others generally being studied at least once in the week.

I learn them via memorising random things, and grammatical example sentences. For the memorized things, I pretty much just write a little note on any word I don't know, sometimes in english, or if I know a cognate of it in some other lang, say German, then that German word. Then I have example sentances, which I don't translate all the way, I just write out the... diffrent-y thing. So like... I don't have much room for notes to write in other langs, as the only thing not in lang I'll write, is more or less just single words, besides the initial notes I make of a language, which... mostly amounts to charts...
I find I can do multiple fine, it's just a matter of getting myself to actually do them, and not get caught up on one for the whole day. :S