NHGIS is awesome for providing high quality shapefiles keyed to each U.S. Census. But these are too high-resolution and in the wrong projection to upload to CartoDB, which is often useful for teaching. This is a Makefile which will convert those NHGIS shapefiles into a format that you can use in CartoDB.
Make sure you have the geospatial tools you need. On a Mac:
brew install gdal
brew install node
npm -g install mapshaper
On Ubuntu:
sudo apt-get install gdal-bin
sudo apt-get install node npm
sudo npm -g install mapshaper
Then download all the NGHIS shapefiles that you want, including unzipping the zip files for individual shapefiles. In other words, all the *.shp, *.dbf, *.prj etc., etc., etc., from NHGIS go in the same directory as this Makefile.
Then run make and you'll get a directory simplified with the simplified shapefiles and cartodb with zipfiles of those simplified shapefiles that you can upload to CartoDB.