How do journals improve their impact factor?
I am an Associate Editor with a journal that has improved its IF from a little over 1 to more than 4.5 over the past decade. Some of the things that we have done to achieve this have been:


Better defining our unique niche within the field. Part of this has been placing an emphasis on publishing papers with finding that are generalisable across sub-fields, not specific only to a narrowly defined group of specialists.
Publishing Position Papers that help set the standards within the field. These Position Papers are designed to help scientists in our field improve their procedures, practises and assessment of their own work. They quickly become standard references, so they receive a lot of citations themselves, but are also something we can point authors to for guidance if their work needs improving to meet the standards of our journal.
When asking for revisions, working hard to make sure the feedback is as useful as possible and designed to help authors improve the impact of their papers. We try to get four reviews of each paper, usually including some comments from an editor about how to make the paper more impactful. The feedback that we have from authors is that, although our review process is longer than they'd like, they do find the extra feedback helps to make their papers better.
Inviting contributions from scientists who we feel can help us to improve the profile of our journal.
Changing the way we handle special issues. Like many journals, we used to let guest editors handle the review and acceptance of papers for special issues alone, with candidate papers taken from a group of invited authors after a workshop or major project. Now, guest editors must work with regular editors who know our standards and what we are looking for, and the opportunity to submit to the issue must be advertised generally, not just to workshop participants. We also make sure the special issue topics are broad enough to be of wide interest.


There are right ways and wrong ways to go about increasing a journal's IF. I think the above points are the right way. Some "wrong ways" that I have heard of include asking authors to cite more of the journal's previously published papers, and coming to agreements with related journals to encourage cross-citations between the two.