Decoupling capacitors: what size and how many?
You need to add a couple of more questions -- (c) what dielectric should I use and
(d) where do I place the capacitor in my layout.

The amount and size varies by application. For power supply components
the ESR (effective series resistance) is a critical component. For example the MC33269 LDO datasheet lists an ESR recommendation of 0.2Ohms to 10Ohms. There is a minimum amount
of ESR required for stability. 

For most logic ICs and op-amps I use a 0.1uF ceramic capacitor. I place the capacitor
very close to the IC so that there is very short path from the capacitor leads
to the ground. I use extensive ground and power planes to provide low impedance
paths. 

For power supply and high current components each application is different. 
I follow the manufacturer recommendations and place the capacitors very close
to the IC. 

For bulk filtering of power inputs coming into the board I will typically use a
10uF ceramic X7R capacitor. Again this varies with application.

Unless there is an minimum ESR requirement for stability or I need very large
values of capacitance I will use either X7R or X5R dielectrics. Capacitance
varies with voltage and temperature. Currently it is not difficult to get
affordable 10uF ceramic capacitors. You do not need to over specify the voltage 
rating on ceramic capacitors. At the rated voltage the capacitance is within
the tolerance range. Unless you increase the voltage above the dielectric breakdown
you are only losing capacitance. Typically the dielectric strength is 2 to 3 times
the rated voltage. 

There is a very good application note about grounding and decoupling
by Paul Brokaw called "An IC Amplifier User's Guide to Decoupling,
Grounding,. and Making Things Go Right for a Change".