First tutorial posting; apologies if there's one similar.
I've been mozeying on through the forums here, and, while most everyone's workflow on sculpting landmasses is similar, I've got a quick, easy and good-looking way to sculpt organic landmasses.
Create a new document. Pixel dimensions don't matter, however, some forethought is needed here. Before choosing a document size, answer these questions:
A) Will the image be used on the computer only?
B) Are there intentions to print the map? If so, what paper size do you intend to print on?
Coming from a graphic design background, I'm a fan of larger documents, because of their scalability. For the most part, I'm not going to have my maps professionally printed, so I'm limited to an 8.5x11 paper size. As such, I choose those dimensions.
Set your resolution to 300 dpi. This is standard, print resolution.
Create a new layer, fill with black.
Filter > Render > Clouds
NOTE: Should you desire, you can render difference clouds a few times, so long as it ends up darker than lighter (it will invert the colors every time you render difference clouds)
Set the top clouds layer to 'Hard Mix'.
This is the point at which my workflow differs slightly from some of the tutorials here.
On the top clouds layer, instead of using a black brush to fill in the seas, use the Burn/dodge tool. (shortcut: O). Dodge will 'raise' your landmasses and Burn will 'lower' them.
I've just started experimenting with using the same workflow to sculpt rivers into the landmass, rather than making a new layer and drawing on it with the pencil tool. So far so good.
When you're satisfied, save the document as a .psd, merge the two clouds layers, and save another copy (DO NOT OVERWRITE YOUR ORIGINAL .PSD). I cannot stress this enough. SAVE your documents at all points before 'destructive' actions. This way, if you want to change the map for whatever reason, you have this point to come back to.