Also realize that bump mode produces rounded protrusions while spike mode produces sharply tapered protrusions. Also, the sphere of attraction tool works much better with denser meshs, so it will help to subdivide areas that will be deformed with it. This is done proportionately. Realize that the offset surface tool can effect any selection of polys to create localized thickness. And also realize that the extrude tool can extrude selected polys perpendicular to their surface at any specified distance towards or away.
Negative numbers specify the opposite direction.
Create objects with minimal geometry from the start, and use selective subdivision wisely. The decimation tool is not predictable, but does work. However if decimate is abused, it can ruin an otherwise good model. If you're working on a head, for example, as is often the case, go to a side view, marquee-select the entire back half of the head, and hide it. The more vertices you hide, the better off you'll be, I think.