Number everything immediately
Assign a number the day a specimen arrives, write it on the specimen or its base in a reversible way, and record it. Memory is not a cataloguing system.
Anything not numbered within a week generally never is.
The fields that matter
Species, locality at the finest resolution you have, date acquired, source, price paid, and any treatment disclosed. Everything else is optional.
Locality should record mine, district and country where possible. A country alone is barely better than nothing.
Keep the old labels
Previous owners labels are provenance and sometimes the only evidence for a locality that has since closed. Keep them with the specimen even when they are wrong.
A wrong old label is historical information. A discarded one is nothing.
Keep the format boring
A spreadsheet with one row per specimen outlasts every purpose-built application, because it can be exported, read on any machine, and understood by whoever inherits it.
Dedicated collection software is pleasant to use and dependent on the developer continuing to exist. If you use it, export a plain copy periodically. Collections routinely outlive the software that catalogued them.