This evening I decided to do a tablet painting of the lovely Samaire Armstrong and save it in stages (partially so you can laugh at how bad the initial drawing is...). I haven't finished it yet, but here's what I've done so far:
Stage 1 - I did a quick sketch to rough in the general proportions, using 3 ruled guidelines to establish the rough height of the top of the head, the top of the eyebrow, and the bottom of the chin.
Stage 2 - I created another layer, and made a rough painting to block in the main areas of colour.
Stage 3 - I created a new layer for the main painting. I copy a section of the original photo and drag it over, so that I can work on it closer (zoomed in) whilst still being able to see the original. I do most of this work with a fairly soft brush, shrunk down to around 2-10 pixels depending on what I'm doing. Working close in lets you get a lot of detail in easily, but it normally means you will have to retouch the image as a whole a lot in the later stages, as it's easy to get broader tonal/spatial relationships wrong.
Stage 4 - I went on to work in the broad detail of the nose.
Stage 5 - Followed by the mouth.