Encoding of abstract concepts in memory
Read "Moonwalking with Einstein" by Foer or watch or listen to transcripts of the world's memory experts, such as Ben Pridmore. You'll notice that they have their own special techniques to make abstract concepts more memorable. These individuals have increased difficulty with less concrete themes, however, and it may be best to process the to-be-remembered items not by focusing on imagery but, perhaps, semantic meaning or another sort of deep encoding.