Changing the selector icon or mouse pointer

I know it must have been hilarious in the early days, deciding to signify the Insertion pointer so that it looked like a capital letter I (for Insert, geddit?).
But the I-beam is not the right tool for the job. I want to change it to a cross-hair, like a gunsight. Thin enough to pick the right time with accuracy, but wide enough to be visible. A plus + all round.
How do I do it?