Mainly you study maps before taking off take note of any unique terrain features, especially close to your waypoints.
To help you you can create a simple diagrams, something similar to this:
Where you basically note distance, headings and time for path between waypoints, additionally terrain features might be drawn there.
To help the pilots there is is usually also manual flight "computers", such as NL-10M, NPL-M. Basically equivalent to the E6B flight computer. NL-10 looks similar to regular slide rule, while NPL-M is basically kneeboard with built in flight "computer", looks like this:
http://gallery.greedykidz.net/get/990229/sf-avia_2011_compressed_ed-DSC_9276-2.jpg?g2_serialNumber=1