The best way I've found to add difficulty to your map is to have friends or the community test it, and take note of places that people will camp, train, etc.
Once you start finding common areas that people stick around because they are "easy," then try to add things like additional risers or choke points like RadimaX said.
I find a good rule of thumb is to make areas smaller and tighter rather than larger, as large areas can make training and / or juking too easy. Of course, don't limit yourself to this. Not all maps have to be small and choke-pointy, and it's good to have a map with dynamic size, but keeping space minimal is a good way to keep people moving from area to area and providing the illusion of difficulty.
Taking my map for example (which is very much a work in progress), you will notice that in the spawn specifically, the areas are very tight-spaced.
It doesn't look too hard at first, but after playing with my friends on it for a while, it definitely becomes pretty difficult.
I hope this helps you out! It really just takes practice. Good luck mapping!