What version of your data should you share, and what file format should you use?
Which version of the data would you personally find useful for a variety of future projects? Generally, the version of the data that you analyzed might be useful in other researchers' analyses, whereas raw instrument data might not be intelligible to anyone who hasn't used that instrument before.
While you often need to work with proprietary file formats during a research project, consider saving a second copy of your final data in a non-proprietary format, to ensure that you and other researchers will be able to access the data in the future.
In general, file formats are more likely to be accessible in the future if they are:
open, with documented standards
in common usage by the research community
using standard character encodings (i.e., ASCII, UTF-8)
uncompressed (space permitting)
Advantages to sharing data in a repository
The easiest way to share your data is to place it in a repository -- that way, you won't have to deal with:
Adding a license to your data.
A repository will either prompt you to choose a license during data upload, or have a standard license for every dataset in the repository.
Establishing access permissions for your dataset.
A repository will have a standard set of access permissions for all files (or subsets of files) in the repository.
Emails requesting data.
Putting data in a repository means that data requests won't contribute to your email overload.
Preserving and migrating your files to ensure ongoing access.
As Jeff Rothenburg said, "digital information lasts forever—or five years, whichever comes first" (quote is from page 2 of his Ensuring the Longevity of Digital Information paper).
Find the right repository for your dataset
Help other researchers discover your data by depositing it in a discipline-specific repository.
Browse the following directories of data repositories to find a repository that specializes in your research area: