One of the biggest mistakes I see people make in business is letting other people control the flow of their day. What usually happens is numerous managers, product owners, project managers, vendors, employees, and executives can unknowingly destroy someones day, and thereby the work that they intended to do.
They don't mean to of course. They just set up a meeting to discuss a few things with a few people, and very likely these are things that do indeed need to be sorted out. The problem is, if you don't control your calendar, everyone else will.
And when other people control your calendar you're very likely not going to be nearly as productive, or effective, as if you controlled it. So while it may seem difficult to decline that meeting, or tell a vendor that you're not available at a certain time, you're actually doing a huge favor to whoever is paying you (whether it's a customer or your employer) if you take special care to control your day.
Likewise, this extends beyond days and into weeks and months. If you're a sales person that travels often, are you really going to give a great presentation if you haven't seen your family for two weeks? I don't think so.
Everyone has certain periods of the day where they're likely to be very productive, and other parts of the day where they're more likely to be checking email, reading blogs, chatting with co-workers, or whatever else we do with our time. And we're all different.
The key is to identify which parts of your day shouldn't be touched by other people.
If you're really productive in the morning, or if you're not a morning person at all and you function better by sleeping until 9, then it's very important that you're not interrupted at 8am. Same holds true if you're most productive right after lunch.
Most people don't ever say no though because they're afraid that they're going to upset someone. In reality, if you tell someone you can't meet at a certain time, they'll most likely just say "okay" and reschedule, or better yet they'll just say that they'll cover for you and bring you up to speed later.
And any meeting where you can be brought up to speed later is a meeting you never needed to attend in the first place. Another key thing to realize is that people are not trying to ruin your day, or interrupt you too much, or steal too much of your time. They're simply trying to include you. Tell them you don't think you need to be included and they probably aren't going to mind.
Control your calendar, control your day, control your productivity. Lose control of your calendar and you lose control of your very self; and no one can function that way effectively.