Make sure that you consistently check the map at print resolution (in gimp you do this by switching off view->View dot for dot). Make a decision about the text size and style on each map and stick to it for all of them. If it's a greyscale map then make sure that you keep an eye on the spread of values by checking the levels. It's very easy to have all the details of an image clustered around mid-grey - especially if you are used to colour work. Ideally you want a good spread throughout the range of values.

Make sure that the important things remain clear - in a map this will be things like the border between floors and walls, roads, coastlines and mountain ranges on regional and world maps, different houses and their labels in a town map. It's easy to get lost in detailing different elements and then not notice that they no longer stand out as different distinct elements at the print scale.

If it's for the clients customers to print themselves, say for a pdf, then check whether they want a printer friendly version. For this you want to keep your lines dark to delineate the different elements, but then pull all the other values right up to the high end - nearly white - so as to save the customer on ink.

If your maps might be used for VTTs then make sure you place the map on one set of layers, any grid on another, any objects (tables, doors - literally anything not nailed down) on another and labels on another. Some publishers will want versions with/without any of these individual sets of elements. Also remember that if it's for pdf printing, they are likely to want a version without DM information on it (secret doors and traps for example).

Finally, remember to sign it, and if it's a jpg then fill in the jpg information (on Gimp this is under the advanced options in the save as jpg dialogue) to include your name, the product it was created for and the copyright information.

That's all I can think of off the top of my head.