assuming you don't have the money or desire to swap out all your paints, I'd undercoat black, then use bleached bone as a base coat. after that do a fairly heavy wash of bestial brown - enough to stain the bleached bone a brown tint rather than just collect in the recesses. you may need to do a few brown washes here.
After that you just need to do the brown patches. ideally you'll use two or three shades of brown for this - there's two methods that can work here - the first is where you just dot the mini liberally with your browns alternating between the different shades.
With the other you do long ragged lines along the limbs and torso. do all your lines of one shade, then the next. When one set of lines cross another, alternate between painting over the existing lines or skipping so it looks like the other shade is on top.
to illustrate the difference in finished minis, take a look at these guys - granted they're in forest camo rather than deseert, but the basic principal is the same.
The 'mech nearest the camera is done in the first style, the others in the second. they were basecoated in GW camo green, and the stripes/dots are goblin green, dark angels green, bestial brown and chaos black
you can see some more pics, mostly painted in the second style here: http://s58.photobucket.com/albums/g267/lorcan_nagle/miniatures/battletech/militia/
EDIT: You mean an aliens marine, don't you? whoops. Well the basic camo design as detailed above will still work, just pick a light tone for your base, then some brighter and darker ones for the spots/stripes. for the armour, I'd drybrush them catachan green follwed by a drybrush of Dark angels green.
