I never underestimate the abilities of experienced pickpockets, people who have been picked can probably relate how they had stuff in pockets, zipped and velcro, that even they had trouble getting into, and they felt nothing.
But then, I also have not had problems. I have pants, regular slacks, that have a normal front pocket, plus a vertical zipper along the seam that is another pocket. My very thin billfold (I normally only carry a bit of cash and a card) and my passport. go in there, my phone in another pocket, but not zipped. And that is it. I would not have a problem using a zippered pocket on the side of my thigh for any of those items, which I do have one pair of pants like that.
If I really need anything else, that goes in a small crossbody bag, but I really try to avoid anything else.
Aside from that, simple awareness of your surroundings is your best defense. If you need to use your phone (for directions or to look up something) step out of the traffic flow, off to a quiet area, do what you need, then put it away. Losing your phone typically happens when you hold it out in front of you, following it around, or casually hanging out of your back pocket (like 90% of the teenage girls I see). The other bad habit (mostly for us Americans) is setting your phone on the table sitting at a street cafe or bar.
If you are distracted, looking all over, except at what is near you, and are loaded with things that can be seen in your pockets, or in an overstuffed day bag, you will be a target. Look like you have nothing to lose, look alert, relaxed, like you know where you are going, you are not worth the effort.