I generally try to have my basing be distinct army to army. It should offer the right effect and the right color-scheme. Sometimes, it's hard to think of how to make things look different. I'll give you a run-down of the basing schemes I have in mind by army:
SM/Dark Angels: Marble Tile + rubble
Eldar: Autumn lawn
Chaos SM: Grey concrete + rubble (adds to the muted effect)
Daemons: Lava bases
Orks: Reddish-yellow sand, rocks and junk.
Tyranids: Barren, brown, lifeless dirt.
Necrons: Desert sand
Tau: Snow + sparse grass
Imperial Guard: Industrial street/sidewalk, occasional rubble
Dark Eldar: Moon-scape
Witchunters/Daemon Hunters: I want to wait until we know if they get separate codexes before I put too much thought into this.
You'll notice that quite a few of those involve rocks/sand; Orks, Tyranids, Dark Eldar and Necrons. Currently I'm working on basing my Tyranids (while painting up another Ork unit for the poll. Damn you guys really like Orks) so I thought I would run a test on some basing sand with a few different washes to see if I could get distinct effects.
The end result was interesting to me, and I thought you all would like to see it. I sectioned it out so you know what wash was used where.
The Devlan Mud is extremely dark, and it's what I went with for Tyranids. I did, however, decide to follow it up with a drybrush of Bubonic Brown to lighten it up just a bit. I think I'll use Gryphon Sepia for the Orks.
What do you guys think?