I've got the same problem you do, and I put up a post under "strings". I think it was called B-string retuning. Several people suggested buying a Hipshot D-Tuner. I haven't been able to try one yet, but it looks pretty good. You replace your E-string tuner with it. There's a small lever on the back of it that you use to tune your E-string to D. You can go to
www.hipshotproducts.com to look at them. I think they're actually called Xtender Keys.
The problem with drop-d tuning is that it messes your intervals up, so the notes aren't in the right places when you go up to the other strings. I'm using a 5-string, and I'm considering tuning the B up to D just for this purpose. That way the top 4 strings can remain in tune. I'm thinking of getting one of those Xtender Keys for this.
Hope this helped a little.