With baseball season starting I expect a lot of moms and pops will be anxious to capture images of their kids in action on the diamond. I don't claim to be an expert on the subject but I've taken quite a few photos at baseball games and learned a few things about the subject so perhaps some tips and example pics might be of interest to some.
If you can get field access, by all means do so -- you'll get consistently better results shooting closer to the action and you'll not have to shoot through the backstop which can get tricky. If you're not sure who grants field access ask the coach or school's sports information director. If you're granted field access, it's a good idea to talk with the head umpire prior to the game and ask what rules apply to you. BTW, don't assume that rules for photogs are the same at all venues and if you're issued written credentials be sure to read them before you go onto the field.
Prior to start of the game you might want to try and get a couple of posed shots of your favorite subject. The classic posed pitcher shot is with them holding their glove hand up to their chest and the other arm down by their side. The classic posed position player shot is with the player down on one knee, their glove lying in the foreground and them leaning on their bat. Both shots look best with either the scoreboard or grandstand in the background.
If you're comfortable shooting in manual mode you'll probably get the best results doing so. If not or if light conditions are changing rapidly AV might work better -- just remember that white or very dark uniforms will mislead your camera's light meter and you'll need to compensate. Check your histogram frequently to insure correct exposure. Maintain a bare minimum shutter speed of 1/350 with 1/500 or faster best along with the widest aperture you can use and still nail the focus. Adjust ISO as necessary to stay within these parameters. Shoot with single focus point and as tight as you can while still capturing the action that you want. Try and get picture with faces as this will make your images more interesting.
The best shooting positions for infield action are from along the first and third base lines somewhere between home plate and first or third base. From these positions you should be able to cover infield action with a 70-200mm zoom. At high school, college and professional levels, with 90 feet between bases, you'll need the reach of 300mm glass for best results. If you want to shoot outfield action your best bet is to go outside the fence and shoot from near the player. Unless you've master the technique of shooting into the sun, you'll probably get better results shooting with the sun at your back.
Pitchers are best shot from their throwing side -- lefties from the first base line and righties from the third base line. Many pitchers have their own unique style of delivery and if you watch them carefully you'll see these quirks -- an unusual windup, stretch, arm angle, facial expression, follow through, etc. If you can capture these split second moments you'll get more interesting stuff and with a starting pitcher you should have 50 or more chances to do it during most games. You'll probably get the best results shooting pitchers in single shot mode and timing your shots carefully. If you have field access and the ump allows it, stand near the catcher when the pitcher is warming up between innings and get a few head on shots from there. If you don't have field access and the backstop is narrow, get these pitcher shots from just beyond the edge of the backstop.
You'll probably get your most interesting batter shots if you shoot in AI Servo continuous mode then keep on shooting after the batter puts the ball in play. Some of my favorite shots are of batters coming out of the box. Baseball is one of those sports where you can anticipate many of the action shots and it's best to prefocus on the spot where you anticipate action -- second base and home plate primarily. Manual focus continuous shooting mode works best for these shots and you might want to stop down a bit to insure that everything stays in focus.
We won't talk about post processing except for a few tips on what to keep and what to cull. The obvious apply -- out of focus and those that are over/under exposed beyond rescue are culls. Unless you need them for lessons learned, delete them and save storage space and processing time. As for the remainder, very few images without at least part of a player's face are of interest. Also (unless you're a fan of the opposing team) photos of position players who make errors and batters who swing and miss or foul tip the ball with the ball still in the frame are usually of no interest.
As I said in the beginning, I don't consider myself to be an expert on the subject of baseball photography so please don't take anything I've said here as written in stone. These are just a few things I've learned in my pursuit of marketable baseball photography that others might find of use.
Here are links to some example pics I shot last season:
pitchers|batters|action & miscellaneous