I use software.
If you can differentiate between eggs (ie: the FBCM went in today, all the blue eggs went in Sunday) then don't bother writing on them. Make a spreadsheet in excel, use a program (I have Brinsea's software) write it on the calender, or even good old fashioned pen and paper and keep track. If I can't tell them apart (I set Guinea eggs every 10 days in the summer) then I use pencil.

If you can differentiate between eggs (ie: the FBCM went in today, all the blue eggs went in Sunday) then don't bother writing on them. Make a spreadsheet in excel, use a program (I have Brinsea's software) write it on the calender, or even good old fashioned pen and paper and keep track. If I can't tell them apart (I set Guinea eggs every 10 days in the summer) then I use pencil.