OP I find it hard saying no as well and always slightly disbelieve people who say they always find it easy. Who hasn't struggled saying no to, e.g. an increased workload when you're terrified of losing your job in a depressed employment market? Or when you just don't fancy going to your well-meaning but tedious colleague's dinner party but they keep hassling you about it, and you have no good reason not to go?
It doesn't help if you come from a family where 'no' just isn't heard - I have relatives who will ask if they can come visit, I say no, and then one day they turn up at my door with suitcases and a giant dog expecting to stay for 3 weeks, pretending we'd never had the conversation where I said they couldn't come, and behaving as through I'm being cruel to their children and mad and unreasonable, and where will they stay when no hotels will take dogs, and I've ruined their holiday, etc. If you were raised by people like this, it can be very hard to set any boundaries at all without tremendous guilt, and a nagging feeling that you might be mad.
Best advice I've had was from my DH, who suggested (re: my family) that I 'behave as though they're normal' - I.e. go into encounters with them trying to expect reasonable behaviour, and feeling properly shocked & outraged (rather than resigned) when they do something mad. Don't just sink in and accept that they can do whatever they want with you because they live in a different version of reality where it's impossible to make them understand.
Also, accepting that it's not your job to make them understand if they won't / can't, and that if they want to be difficult then you can be difficult too.
Of course this is all more complicated if the person you're trying to say no to is your boss, or a very important client, etc. I think that's part of what people mean about it getting easier as you get older, as with any luck you have a bit more power and autonomy as you progress through life & career.