If you build yourself a library of your commonly used objects creating maps will be faster.
I don't use inkscape in particular, but in other programs, you can import another picture from the same program, and use as an object inside it.
For example, you decide that you're going to need a forest, so you open a new picture and draw a tree (or maybe a few trees).
Then you go back to your original map, import the trees picture into a new layer (like Jerry Stratton says, layers are incredibly useful), and duplicate them to rapidly make a large forest.
If you don't like duplication, you should then be able to add bits to modify some of the duplicates (if it's looking too regular).
It's also useful if you're making lots of maps, say you have a signpost picture in one map, you might want to reuse the graphics in a different map (replacing the text).