Icehouse: Remembering Associative Entities gives a nice rule for how to do associative entities. Icehouse generates a lot of thes handy rules of thumb.
1) Picture/draw the ER diagram Many-to-Many relationship between the two entity types (without the associative entity). 2) Add the box for the associative entity inbetween them. 3) Move each of the two cardinalities that are next to the two entity type boxes so they are on the opposite sides of the associative enitity box (in other words, next to the associative entity box but on the other side of it). 4) Put 1:1 cardinality next to the boxes for both entity types