How to make research software citable ?

An interesting paper on that subject :
Laura Soito, Lorraine J Hwang, "Citations for Software : Providing Identification, Access and Recognition for Research Software", International Journal of Digital Curation, Vol 11, No 2 (2016), doi:10.2218/ijdc.v11i2.390. http://www.ijdc.net/index.php/ijdc/article/view/11.2.48/451

The two main solutions are :
1) writing a "software paper" and generally deposit your software in the journal repository,
2) or just obtaining a DOI (Digital Object Identifier) for the software.

Publishing a Software paper

This page lists journals where you can publish your software :
https://www.software.ac.uk/which-journals-should-i-publish-my-software

If your software is domain specific, you will probably find the suited journal.

if your software is more domain independent (a computing library for example), you can look at :
 the Journal of Open Source Software. You fill metadata about the software and you write a short paper. After a pre-review process (dialog with the editor, search for reviewers), the review process begins. It concerns mainly the software and its documentation. Note that the review is an open process (see that example). When your paper is accepted, you obtain a DOI for the paper. But you are also asked to obtain a DOI for the software, using for example Zenodo in GitHub. No publication fee. Open Access. You can read "Journal of Open Source Software (JOSS) : design and first-year review" published in December 2017.
 the Journal of Open Research Software. A fee is encouraged but not compulsory. Read this author review of this journal.
 Elsevier SoftwareX. Open Access with a 500 USD publication fee.
 Hindawi Scientific Programming. Open Access with a 1750 USD publication fee.
 Computation : free access but with a 1400 CHF publication fee.

 ...

Obtaining a DOI

 With Zenodo for example. See the tools proposed by GitHub : https://help.github.com/articles/referencing-and-citing-content/