I think basing models makes such a big difference to how they look on the battlefield and aspiring toward having realistic good-looking models is one of my top priorities.
I've gone through many different trials for getting the perfect base colour over the years but it's only this one I'm really happy with. Even so, it does fail in one crucial way: It isn't transferrable to a snow-covered board. That is a bit of a shame.
Still, here's how I go about it, using the nice big base of the Arachnarok Spider as an example. Obviously the same principles apply to smaller bases and I follow these exact same steps then.
Here are the materials I use. In the big tub is Bird Grit. This is a product available in pet shops and Wilkinsons that is amazingly cheap compared to GW gravel (about 80p for a bag bigger than the palm of your hand).
In the smaller tub is fine beach sand.
Using black paint I paint the base then place it in the tub and push the gravel onto it. Sometimes it doesn't stick that well. In those cases, adding another layer of paint can help or using the beach sand to fill the gaps.
It's important to leave every stage long enough to dry or it messes things up badly. What I often do is use a drying lamp - basically a desk lamp pointing down at the miniatures. This speeds things along. A nice summer sun in the garden also works, as we can see from these pictures.
When the gravel is dry, paint over the whole thing with black paint.
Next up, give a thick wet-brush of Scorched Brown across the whole base.
From this point on things speed up as we're not applying too much paint at once so drying times are cut.
The next thing you need to do is dry brush patches of Bestial Brown across the base. Leave some of the Scorched Brown showing through.
When that's dried, do the same thing with Bubonic Brown - a rough overlapping series of patches that allows all the colours already on to show through in small areas.
At this point your base should be a mottled mix of patchy colours ranging from black through darker and lighter browns up to yellow and cream. It think it looks quite interesting and certainly blends very well into the warboard if that is painted following the same steps.