I love going to the ballpark.
Unfortunately there have been times when an incessant flow of late fans, vendors, and people who apparently haven't eaten in a week and need to take a trip to the concession stand every inning have ruined my experience.
Depending on the stadium and the quality of the team, you may often find yourself surrounded by people who would be hard pressed to name 5 players on the home team. What that means is, they really don't care what happens that night at the ballpark. The fruits of that include showing up in the 4th inning and leaving in the 7th.
Another part of that is the tendency to casually stroll up and down the aisles and block the view of fans who are trying to watch the game. If only everyone was courteous enough to wait until between innings to walk down the aisle. Because this probably won't change any time soon, here are some things you can do to make sure your great view is safe from pedestrians.
1. Sit Behind The Plate
When sitting in the upper deck, there is more than one advantage to sitting in the section right behind home plate. Envision that you are right behind the plate; it will be difficult for people in the aisles to block your view of the action. This is especially true if you are sitting in the middle of the row. While upper deck seats behind the plate are traditionally a few bucks more than there down the line counterparts, I would contend that it is worth it in most cases.
2. Avoid aisle seats closest to the plate
Imagine you are sitting on any level, between first base and the right fielder. If you get aisle seats closest to first base in that section, you will have an unrelenting flow of traffic in your face for the majority of the game. Up until the 4th inning it will be confused fans and an accompanying usher helping them find there seats because they have just arrived. Starting in the 6th you will see people head for the house no matter what the score. Combine that with bathroom and concession trips along with vendors and you have missed half the game. (Let me note that this hasn't been my experience at every stadium. Stadiums with a high level of intensity and a reputation for knowledgeable fans do not have this problem nearly to the same degree.)
In my example above, a better choice is to request aisle seats on the side of the row closer to right field. You will still have people walking by you, but this time they will be on your right and the infield will be to your left. God help you if a ball gets hit to the right field corner, but at least you have a generally unobstructed view of the infield where most of the action happens.
3. Sit in a low row
Again, sounds obvious but I want to convince you why sitting in row A is more important than you might think. (Please see my stadium specific guides for instances where row A might have the opposite effect). Sitting in row # 1 on the field level is not something that you probably do frequently, but sitting in the first row of the upper deck is usually doable. The concept of what I am saying revolves around the fact that the less people that are in front of you, the less people that will stand up and move in front of you. This is especially true on lower levels where the concourse is located at the top of the section. Everybody is going to get up and walk back to go to the bathroom, so if you are sitting in the back row there is a potential for everybody in front of you to walk back and block your view for a moment.
In most if not all upper decks you have to walk down to get to the concourse. At many stadiums, like Great American Ballpark in Cincinnati, the view level is elevated so those in row A don't have people walking back and forth in front of them. These are also ideal in Cincinnati because people making the trip up the steps will actually enter the upper deck behind you and never block your view. You may think I am making a mountain out of a proverbial molehill, but it makes a huge difference in the experience – especially if you care about baseball.
In my Cincinnati example, I would move from the behind the plate to down the lines in the upper deck if it meant I could sit in row A. If I can't sit in the first 3 rows of the section, you will then find me near the top of the stadium. Once you are that high, what is the difference in row M and row Z? Not much, in my opinion. The benefit of being near the top vs. in the middle is just that less people will walk up past you, so you can have fewer obstructions.
Hopefully this gives you some ideas. Remember that not all cheap seats are created equal.