There are many good tutorials on the net on how to make good asteroids. Many techniques are very similar to others. This is a technique I use - and it's to be honest similar to most of the others that there are out there.

Start off by making a box in Modeller - a simple box of any dimensions with settings at default - i.e. no further segments - Then subdivide this box using the metaform option with the values at default - One point here a smoothing angle of 90 or less won't do much if anything at all to the resultant subdivide - with an angle of greater than 90 degrees you'll get something like this:

Now - repeat this process once more but this time specify a value of up to 2.0 in the fractal box - Fractal basically applies a random jitter to the new points created. Now the jitter radius is a fraction of the edge length. A value of 1.0 will jitter the new points by about 50% of the length of the edge that they'll subdivide. A value of 0.1 will cause a 5% jitter and a value of 2.0 will cause 100% jitter or the full length of edge being divided.

One thing worth mentioning is that subdivide will generally divide 4-side poly' s into smaller 4-sided poly's.

Now you should have something like this:

You can do it again for more detail or stop right here and then hit the TAB key to metaform the object. Now use the control points to deform your rock to your taste:

 

Right now at this point you can freeze the object - Remember that before you freeze the object you can hit the "o" key for options - which will show you this options screen

 

You can set the mesh to be made from purely Quads or Triangles or Automatic - where Modeller will make the mesh using Quads and Triangles in optimum places. The other main box we're concerned with here for now is the patch division one -- A value of 1 will create relatively few poly's in the final mesh - a value of 2 will be better: See below

Patch division Value = 1.0

Patch Division Value = 2.0

At this point apply suitable surface with a good smoothing angle - say about 60 degrees and if you're happy it's ready for texturing.

To apply a little more detail undo to before you froze the object and then set the options to create an automatic mesh with patch division value of 2.0 and freeze it again. Then apply a jitter to the object with the options set to scaling and a factor value 0f 12% - you will get a horrible mess like this:

to tidy everything up apply a metaform again with a patch division of just one and freeze - now onto surface smoothing

Smoothing angle 45 degrees

Smoothing angle 80 degrees

No Smoothing at all

Texturing your asteroid is really a matter of taste: And as such the texturing can be pretty powerful - so much so that realistic effects can be achieved with simple meshes