Basing can make or break a model. Even if your painting skills rank only a "meh", complete and uniform basing can make your army really pop. But if you're looking to make your army look super-sharp, complex or creative basing can be one of the most important steps.
I recommend devising a theme for your army. For example, I thought it'd be fun for my Dark Eldar to be assaulting a Mechanicum Forge World. If you've read Graham McNeil's "Mechanicum" or Dan Abnett's "Titanicus" then you know what a Forge World looks like in my mind's eye: dusty, gritty, utterly polluted, littered with scrap machinery and junk parts left to rust for eternity. My Dark Eldar would be raiding this Forge World to steal items and technology to sell to other races or to use for themselves, as well as grab a few Tech-Adepts and menials for the torture tables! Therefore, they'd be blowing holes in buildings and assaulting through to nab all the cool stuff. I itemized what I wanted to do, and wrote down how to do it.
1: Dusty/Gritty: The obvious starting point would be gravel/sand glued to the base, what really mattered was the color choice. The sand would be painted GW Vomit Brown and then dry-brushed GW Plague Brown, with a few spots of dry-brushed GW Bleached Bone. I also added stones and boulders buried in the sand, using my old standby of cork glued to the base before the sand. Here's the type of cork I use: Linky-link!
2: Junk: I glue anything to the base that looks even remotely mechanical. My favorite are lock washers because they look like cogs or gears. Plastruct and Evergreen make a variety of useful types of plasticard rod such as I-beams or ladders, and all sizes of round tubing. GW bitz are also super-useful. I've been using the inside gubbinz of Space Marine Drop Pods, (because I don't use them on the inside of the Drop Pods, I just glue the SoB's shut!) because they can be easily cut to look like ubiquitous machine parts. Searchlights, jerry cans, ammo bins, IG stowage, grating cut from SM Razorback hatches, exhaust ports, anything! A $20 box of Cities of Death terrain can supply an entire army's worth of basing material! Just start digging through your bitz bins until you can find something that could be mistaken for machinery. Even just brass rod bent at rounded 90 degree angles can simulate laid wiring.
3: Buildings: Of course I can't construct buildings on bases, but I can hint at them. Remember, the miniature is a little display of another reality plucked and placed onto a base. It's perfectly fine to have just a portion of something showing on a model's base. Once again, I return to Evergreen and Plastruct for plasticard sheets with diamond plate patterns, tile patterns, corrugated sheet patterns and siding patterns. I like Evergreen's 1/4" Tile pattern plasticard because it has a very defined cut, snaps easily into workable pieces, and looks just like a tiled or paneled floor! I add pieces of standard plasticard sheeting to simulate a broken wall, just leave on edge flat to glue to the base where the tile stops, and rag up the other edge with snips or a knife.
4: Pollution: This mostly came down to choice of paint. Having "oil" leaking from pipes or into drainage ditches by layering on green, brown and black washes to flavor. Most of the metal things were painted with splotches of rust, using a progression of GW Bestial Brown, VGC Hot Orange, then VGC Fiery Orange in a stippling pattern.
I suggest gathering your materials before you start your project, and make sure you have enough for the whole army. Take an hour and go to your local hardware store and check out their supply of washers and nuts, something is sure to catch your fancy. If they do window screen repair, ask for some scraps of aluminum screen, it makes great decking and grilles. I bought a piece today from Grieve's that was 12"x 24", it cost me $1.10, and it's more than I'll ever use. Stop by Hobby Lobby or Michael's and just look for doodads. Most of the stuff can be picked up cheap, and it's dead-useful.
So, I'll leave you with some shots of my DE bases, and I'll be back next time with some more basing techniques ,or whatever I damn well feel like!