I sent mine a thank you letter. My recs are quite spread out, time wise. I got on last spring during my senior year, I went and asked another prof for one this spring, and I'm asking for one from an attorney I work for now.
I sent thank you letters to my professors since I don't see them often any more. I may buy my attorney a gift certificate or a bottle of wine or something after he sends the rec in (so it doesn't look like influencing).
I also think that, in Dec, Jan, Feb, when ever it is that I know where I'm going, I'll pick up some memoribilia from the school for each of them, and send it on, thanking them again for the recs and letting them know what my plans are, where I'm going, etc. That way they get to see that they did, in fact, help me by recommending me, and they'll hear again how grateful I am. Best of both worlds, IMHO