A really simple but effective basing technique is to use builders aggregate instead of sand (it looks more like rubble than most sand does), black undercoat and then drybrush with a dark or mid-tone Gray such as the old Codex Gray. Then add small bits of static grass. You want a dark grass though, rather than a bright green. I use marsh static grass from Gale Force 9. It has little blood red bits that resemble poppies in there to add a slight bit of colour to what would otherwise be a very dark base.
Here's a pic.