So here is a systematic review (this is the best way of analysing all the studies done on a particular subject) on the evidence for fibre in treating constipation: http://www.ncbi.nlm.nih.gov/pubmed/21332763 It concludes:
Soluble fibre may be of benefit in chronic idiopathic constipation, but data for insoluble fibre are conflicting. More data from high quality RCTs are required before the true efficacy of either fibre type in the treatment of chronic idiopathic constipation is known.
('Chronic idiopathic constipation' just means constipation that has been going on for a while and for which there is no obvious cause).
So, just increasing fibre may not be the answer. If you do want to increase your fibre intake it looks like you should concentrate on soluble fibre, such as oats, chia seeds, pulses (legumes) rather than things like wheat bran, flaxseed etc. This is contrary to what we have generally been told (much like the low fat story, eh?)! Indeed, I found this interesting review of the causes of constipation: http://www.ncbi.nlm.nih.gov/pubmed/17219073, which says:
A search of recent literature, however, reveals that there is a paucity of evidence-based publications that address the etiologic factors of chronic constipation. Much of current writings on the subject may be based primarily on myths handed down from one generation to the next.
This review http://www.jabfm.org/content/24/4/436.long suggests that the general advice handed out by all doctors (i.e., more fibre, more water, more exercise) only works if the patient has deficiencies in this area, so if you already have plenty of fibre, water and exercise, increasing them will not help. Interestingly, I found a statement in this paper saying
One prospective study showed that Bifidobacterium supplementation can relieve constipation in hypocaloric dieting,95 whereas another prospective trial reported efficacy of Lactobacillus in improving chronic constipation in nursing home residents. However, survival and viability of these probiotic bacteria as consumed in commercial form has yet to be standardized for such treatment to be officially endorsed.
This shows that constipation is perhaps an inevitable consequence of just eating less, but that it might be possible to improve things with probiotics.
Regarding prunes, I found a randomized controlled trial (the best kind of scientific study) looking at prunes vs psyllium husk, which came out in favour of prunes: http://www.ncbi.nlm.nih.gov/pubmed/21323688
Looking at increasing fluid intake, I found mixed results of studies. My general impression is that if you are dehydrated this can worsen constipation which is then improved by drinking more but that if you are adequately hydrated anyway, drinking more may not have an effect on constipation. I did come across one theory that if your drinking water is high in magnesium sulphate (i.e., you live in a hard water area), this could improve constipation and so the water hardness could be a confounding factor in studies examining the influence of water intake on constipation. (Magnesium is a well-known and widely used laxative, and as magnesium has so many other benefits, particularly on sleep, it's the method I use to ensure regularity!).
So, bringing all this together, I think the best approach might be to create your own breakfast cereal with oats, chia seeds and prunes, serve it with natural yoghurt and at the same time ensure you are drinking enough throughout the day. But for me, I'm sticking to my magnesium!
Finally, here's a great page summarising the evidence for different treatments for constipation: http://www.jabfm.org/content/24/4/436/T1.expansion.html