It's not hard :) I use PSP X2 and set the resolution when I start the image. Then I just recheck it when I'm done to make sure it's still where I want it. Pixels per inch and dots per inch are the same thing.

The stuff for my shop needs to end up at 300 DPI ,so to get a better quality product I just got in the habit of working in a higher resolution. That way I can resize the images to more templates without losing quality.