I start by asserting a lot of conditions in my code (in C#, using System.Diagnostics.Assert), but I add logging only where I find, while debugging or putting the system under stress, that I really need to have a way to follow what's happening inside of my code without having a debugger permanently attached.
Otherwise, I prefer using Visual Studio's capability to put traces in the code as special breakpoints (i.e. you insert a breakpoint and right-click it, then select "When hit..." and tell it what to display in that case). There is no need to recompile and it is easy to enable/disable the traces on the fly.
