When should I get recommendation letters?

I am a sophomore biology/pre-dental student, and I was wondering when the best time is to start asking for recommendation letters from professors. Can I ask a professor at the end of this semester to write me one? Or do I need to actually wait until I apply? My concern is that by the time I apply, the won't remember me as well. Also, is asking a professor I had freshman or sophomore year considered not recent enough when I eventually apply?

Thanks

Nothing like stale/outdated lors.