Organic modeling is considered by some the most challenging. Non-organic can mostly be accomplished by extrusion and scaling. Organic modeling on the other hand involves mainly curves, as nature has a thing against straight lines. Because of this, organic modeling is usually done with subsurfaces. To subsurf a mesh, first select it and navigate to the 'Editing' tab. Then go to the modifiers stack and add a new Subsurf modifier.
In Blender 2.5 Alpha 0, Subsurf has been removed from the modifiers list. Instead hit Ctrl, followed by a number which will specify the subsurf level. For example Ctrl-1 adds a new Subsurf modifier of level 1. This also works in Blender 2.4. Increasing the level greatly increases the number of verts in your model, so make the level relative to the number of vertices in your original model (pre-subsurfed). Subsurf often works best in conjunction with smoothing, so be sure to set your object to smooth, again in the 'Editing' tab, or in 2.5 A0 under 'Object Tools', which you can bring up by hitting the T key in the 3D viewport.
In this way you can create a smoother, higher-poly shape based on that of the original mesh. This is controlled by the vertices, edges and faces of the original.In order to control the rounding of your mesh you can use two methods: loopcuts, which is a very sloppy method as it adds more verts to your mesh, which serve very little purpose and can get in the way of modeling; and Edge Creasing. The latter can work extremely well in most circumstances. By default all edges are declared uncreased, and so allow complete rounding in subsurfing, by creasing the edges using Shift-E you can far better control the amount of rounding on your mesh, and most importantly, without adding extra vertices. However, unfortunately Edge Creasing is not available in Blender 2.5 A0. Yet. And so loopcuts and extra verts are the only option.
Now, start shaping your mesh into an organic shape. There are various tutorials for modeling bodies and faces, but it is often a good idea to use references. Only use quads - that is, shapes with four vertices. Triangles do not subsurface well, they create bad looking rough surfaces - avoid if at all possible. Also avoid Poles, which are vertices connected to anything other than 2 or 4 other vertices, as they create literal creasing in your subsurfed mesh, that is, sudden bunching up and pinching of faces.
To make the object shape properly, you will need good edgeloops and edgeflow. This means that all your vertices and/or faces line up in a continual line or curve around your model. It is possible to select edgeloops all in one, fantastic when you have to resort to them in controlling rounding, by holding down Alt when selecting a vert. This really expands into topology, which is an advanced subject. I would recommend visiting http://www.subdivisionmodeling.com/forums, although it has just been closed down, which was a great place to start, however it still (at the time of revision) is active and well worth a look if you are struggling with modeling heads. It is best not to have too many vertices to avoid making the subsurfaced shape look rough - in other words, the original shape should be quite low-poly.