The comments field for each species in an eBird checklist allow you to describe the details of your observation, including the points that led to the identification of rare species.   You can also embed photos you took of a bird to help support your identification.  For particularly rare sightings (that are not likely to be disturbed by visitors and in public areas), you might consider also adding detailed directions to a bird so that others can go find it again.

Zachary DeBruine, at birdventurebirding.com, has developed a simple tool that will allow you to add a very specific image from Google Maps detailing the exact location of a bird you have found.  This can be a big help to others that go to look for the bird.  For example, here is the location of the exact Himalayan Blackberry bush at Lytle Ranch where I found a Harris's Sparrow a couple of weeks ago.

 
Generated by eBirdGM

When this image is embedded in a checklist, it includes a link to the location on Google Maps.  So if you wanted to look for this bird, you could get directions from your house to this specific shrub with just a few clicks!

Here are a few sample checklists with embedded maps:
Harris's Sparrow, Lytle Ranch
Blue Jay, Smithfield
Dickcissel, Farmington
Greater White-fronted Geese, Logan Polishing Ponds

For thorough directions on how to use this tool, and for the tool itself, see eBirdGM.