If you have a domain checking tool, such DNA Pro, create a list of your own domains and scan it regularly.
This will help you track what you have and where it is parked / hosted.
Check every domain has Nameservers defined by sorting the list by nameserver, this this help you to see any domains you forgot to park or host somewhere.
If you use Sedo you can export the revenue stats into Excel easily from their site, check that everything pointing to Sedo nameservers is actually in your account, otherwise you wont be getting revenue recognition for it.
As domains come up for renewal, check to see if they made their reg fee back over 2 years and then decide to keep it or not, some are worth keeping on the basis they may sell some day even if they make no direct revenue.
If you use 123-reg, make sure you look at any domains coming up for renewal as they will automatically renew them unless you go into your control panel and release them.
It takes time and effort, but check you portfolio at least once a month, I always find something I forgot to do!
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