You forget details over time and it's incredibly difficult to reuse old data without documentation.
Don't forget to back up your written notes! This is especially important for graduate students whose PIs keep their research notebook. Scan or photocopy your notes and keep them with your data.
Files using proprietary file formats are more difficult to use over time. Convert your data into an open, non-proprietary format that is in wide. This will improve the odds on reusing your data in the future.
Keep a copy of your data in both formats. The original file lets you use all of the file features and the open copy is a backup in case something happens to the software.
This table lays out some file format recommendations:
|Tabular data||.csv, .tsv|
The sad fact is that hardware fails and becomes obsolete (think: floppy disks). Therefore, you need to periodically move your data to new storage media.
Plan to update your storage media every 3-5 years. Move your data to the current technology to avoid falling behind.
The easiest way to care for old data is to let someone else do it for you. This is one purpose of a data repository.