Slow draining bathroom sink

I have a problem sink. Our master bathroom sink is always causing me fits. Usually it is a foul smell that just shows up randomly. I've been told that it is either a dried up P trap, or something foul in the overflow tube in the sink. Either way, that is not the issue right now.

Every so often it drains slow. Usually because I trimmed my sideburns or over my ears after clipping my hair, and some of it goes in the drain. Sometimes it is just a buildup of my wife's hair from making herself beautiful every morning. One time I took the P trap off to find a half moon sliver of soap/toothpaste/scum/calcium had been growing in the bottom.

Well a couple weeks ago after I had washed some hair clippings (not a lot either, I'm not a total idiot) down the drain, it started draining really slow. I mean not like minutes to drain, but slow enough that you couldn't leave the water running for very long. When you were brushing your teeth your toothpaste spit would just float around, pretty nasty.

So I took the trap off, and it was surprisingly clean. I pulled the drain plug out and UGGGHHH. Super nasty! I cleaned if off and using a hanger scraped the top portion of the drain itself clean. After that it was flowing perfectly. Until a couple days ago.

Slow draining once again and no real idea why. Any suggestions would be appreciated, and when I figure out what is causing this (probably later tonight) I'll post it. But I need to know how to really prevent this altogether! Garbage disposal in the bathroom??? They need to make a toilet with one of those, then we wouldn't need plungers.