Keeping track of sources can be challenging and time consuming - worthy of mention here. N.B. This has nothing to do with licensing (addressed elsewhere) - just good academic practice, and there may some tools which help one to keep track of attributions.
Agreed. I've added a subsection on keeping track of sources. Essentially, I recommend using a spreadsheet.
When remixing media files (e.g. images) it can be useful to store them in folders "CC-BY", "CC-BY-SA", "GNU FDL", "PD". Then each folder could include a spreadsheet with attributions. At first, just the URL of the source for reference, and when actually using an image, obtain the correct attribution from the URL (add it to the spreadsheet, and include it in the OER's list of attributions).