How to suggest reviewers for a journal? I recently tried submitting a paper to a journal. It was mandatory to suggest three reviewers. Is this a norm in journal submissions? If yes, how should one choose reviewers if I do not personally know any experts in the field? I have been submitting papers to conferences and never found such conditions there.
Being editor of a journal where authors can provide preferred and non-preferred reviewers, I can provide some "inside" thoughts on the subject based on what has happened in "my" journal. Note that it is possible to suggest names for review but also provide names which are not preferred. The latter can be because of a scientific disagreement, personal issues or whatever. Such suggestions appear but not often and we usually follow the suggestions (not that we have to!). 

When it comes to the preferred or suggested reviewers, I have been tempted to use such reviewers on occasion when it has been hard to identify reviewers directly. Sometimes because the topic is local and where it would make sense to have local input. In these cases, I cannot remember a single reasonable review that has come out of such reviewers. This can be for several reasons but most often the review is a close colleague who might have an incentive to help the author. In some cases the preferred names have been very senior scientists who, I am afraid, has lost touch with the subject and provide poor and in some cases almost non-existent reviews. Out of all immediate "Accept" review recommendations I get, the vast majority have come from these reviewers. So, I not longer trust these names and avoid them at all costs unless I personally know or know of the reviewer and his or her good reputation. 

In addition to what I just describe, I also must state that it is often the weakest manuscripts that have listed several suggestions. This can be identified by the disparate review results, sometimes one accept (by the suggested reviewer) and one reject. 

Now, in principle, there is nothing wrong with suggesting reviewers, I have done so myself when being requested. I have then as a principle gone for established and well renowned names in the community. The problem lies in suggesting names for a purpose other than to get a fair and objective review.

It is clear that the system can and is abused and since I became Editor-in-Chief, I have come to rely less and less on these suggestions and now mostly look upon them with suspicion and make selections from my own understanding of the field and investigations into the subject literature. The best suggestion, I can provide is to not avoid mentioning names but pick names that in your opinion can provide good constructive critique on your work (and not just favorable).  A note on why you have selected names as preferred or non-preferred would greatly help as well since it puts your choice in a perspective.