or do you just put the next beer right on top of the last beer old hop debris and all.
Sometimes, yes. You can do this and the typical approach is to go from lighter/weaker to darker/stronger beers (e.g. brew a light lager, use the yeast cake for a dopplebock).
Or, you can pour the yeast cake into a sanitized container and wash the yeast as mentioned above.
Or, you can pour it out and use only a portion of the total cake for your next batch which minimizes concerns about over-pitching.
There are several ways to go about it. Find out which works best for you and go with that.
+1. If you are really worried about the amount of trub, then the wash is the best way. I have found that it really doesn't make a difference. I typically fill 2 1L containers half-way and just pitch one entire slurry in my next brew. If you want to pitch on top of the cake, that's fine, but like Joe said, make sure it's to a much higher gravity beer.