Hi Roc, depends on the brown. If it's close to a tube colour or I know it can be based directly on one (buff = BU + white for example) then I'll use it for speed and simplicity.
With a few exceptions browns are basically dark orange-yellows through to orange-reds so you can mix just about any brown from a yellow, red and something to darken and/or dull it. Black is useful for this but often won't lower the saturation quite enough in which case you have to resort to using a complementary colour, Phthalo Blue GS is ideal for this but most any dark blue will work fine, even French Ultramarine which is as different in hue to the other blue as you can get but it still works okay.
As Francis mentioned you can also mix reds with greens, many artists make their browns this way, any combo along these lines is worth experimenting with and you can get some very interesting colours that you're not quite expecting. If the red and the green are close to being mixing complements at about the balance point between them you'll be close to grey - loosely called neutrals by painters - which are some very attractive colours in the right context. Almost all green paints are transparent to some extent though, so bear this in mind if you want a mix with good coverage, in which case use Chromium Oxide Green or mix your green from an opaque yellow and blue and/or Mars Black.
Einion