When you want to release your work to the widest possible audience, and inform other people that they can re-use it under certain conditions, consider a Creative Commons licence.
By adding a CC mark to your work, you're choosing to waive certain aspects of your copyright (such as the right to benefit financially, or to prohibit reproduction), without relinquishing your ownership of the work.
Breaches of your licence are not automatically detected, but you will be in a strong position to approach anybody who you discover using your material in a way you have not endorsed, to ask them to stop (threatening legal action if necessary!).
Source: Pixabay, CC-0
The Creative Commons Licence Chooser guides you through the key questions to consider when deciding which licence is right for you, and provides code to embed the CC graphic in your work.
JISC has published a guide to Creative Commons for Humanities and Social Science Monograph Authors (2011) which also provides useful background for other disciplines.
Software authors can take advantage of the GNU General Public Licence to assert their right to choose how their work is distributed - either free or charged for - and to require other users of their work to respect these terms.
The UK's Digital Curation Centre has published a guide (2014) to licensing your research data.