REIT's are also already a component of the S&P 500, so keep that in mind if you choose to pick some (more) up.
For me, I don't really bother with that much extra complication and stick to my total market indexes, but I know some people like to goose their portfolios one way or the other by adding a bit of say, small cap, to their main total market fund holding. I do hold a bit more international right now, though (40%), as I believe there's some value to be had there.