The general rule of thumb is that you should be eating about five "portions" of fruits and vegetables a day, preferably different ones (i.e. not five bananas). However, at some point I read an article that said that while five is sufficient to derive most of the benefits that research shows you can, up to ten is even better. There are a ton of articles online about this stuff so I am not going to bother providing links: just google it and you'll find loads.
So at some point I decided to take this seriously and find a way to increase my intake up to the ideal of ten. And I've been doing this in the following way: Every day when I wake up, I make a list with ten slots. When I eat a piece of fruit or vegetable, I write it in a slot, slowly filling up the list throughout the day, with the goal of hitting ten. If I eat the same thing twice, I don't take up another slot for it; I write "x2" next to the first slot I used for that particular fruit or vegetable.
I've only hit the fabled ten a couple of times so far, even though I've been doing this for months. I usually hit about five, often six, and sometimes seven. I rarely manage to go above, but it does happen, and always on days where everything goes well for me, because if I have time to bother obsessing about the exact number of fruits and vegetables I've consumed, it must mean that I am acing all other goals/tasks that I have set for myself that day. Here, for example, is what my list for today looks like right now:
4. Orange + lemon juice
I still have dinner left, and maybe a snack after that, which will include at least one fruit, so I will probably get up to about seven today. Maybe eight. Which means it's a good day—and as I just explained, not only in terms of diet, but also in terms of everything else, otherwise I wouldn't have bothered taking such great care with my eating.
Now, remember, that if you are eating five, you are doing well—that's what most doctors recommend—and since I started this thing my average has definitely increased considerably above five (I would say it's around six, whereas before it might have been four or even three), so you don't have to hit ten every day—or at all—in order to see the benefits. Merely by doing this thing you will be giving your diet a huge boost. And by the way, the couple of times I've hit ten I've felt wonderful. I don't think it's placebo effect either, because when I don't eat fruits and vegetables AT ALL for a couple of days I start feeling like shit, so it would make sense that when I eat A LOT of them I'd feel the difference in the opposite direction.
Another thing is that, generally speaking, vegetables are more nutritionally valuable than fruits, so you want to try and eat more of the former than the latter. At the start, don't bother thinking about this, but as the list starts becoming routine, and your numbers climb to respectable sixes and sevens, you'll want to start thinking about replacing those lazy bananas with stuff like broccolis and carrots and so on. Fruits tend to be simpler to prepare (usually there's no preparation involved at all beyond washing), so by all means use them to get you started and give you a head start, but as you become good at this you'll want to take it to the next level, and vegetables are where it's at.
And of course buy organic if you can (I do so when I am in a civilized country like Sweden, but in Spain, and especially the Canaries, they are not so readily available), and either way wash everything thoroughly, etc. I even wash the oranges before I squeeze them (because the chemicals get on your hands and drip into your drink while you squeeze them). You should be washing the organic stuff too because, 1. Dirt is dirt, and 2. Some organic stuff is falsely advertised as such, so you can never be sure.
In the end, good practices are about good habits, so knowing HOW to form good habits is an extremely valuable skill. This "gamified" method I devised to increase my intake of fruits and vegetables can completely change the quality of your diet if you take it seriously (and I have introduced it in other areas of my life too, which I'll be discussing in an upcoming post). You'll go from someone who only eats two-three types of lazy fruits per month and a couple of vegetables to someone who spends an additional half an hour at the supermarket trying to find precisely TEN fruits and vegetables to fill his basket with (and it's better to avoid supermarkets and go to specialist produce stores for higher quality stuff and greater variety). You'll have to LEARN about NEW fruits and vegetables you've never eaten before, in order to have a hope of hitting the target now and then. You WILL become a better cook as you seek out ways to integrate all this produce into your cooking instead of just eating it right off the supermarket bag to score your extra points (though note that with rare exceptions such as broccoli, most of this stuff must be eaten raw to derive the maximum health benefit from it, but there are of course cool salad recipes which can be used to combine items in varied and delicious ways). And of course your taste in food will improve as well, because all these new and nutritionally dense fruits and vegetables you will be eating on a regular basis will exercise your faculty of taste far more than the fast food-type trash in your diet that it will be replacing.
This little game can completely change your diet for the better in the long run. Try it, stick with it, and you'll see.