When thinking about what to do to your field, the first thing you should look at is the dirt level of the skinned area. More than 80% of the fields we look at need dirt (infield mix). Fields that are low on dirt drain poorly. Good dirt will be noticed by the players; adding dirt will resolve low spots on a field that cause erratic ball behavior, and wet spots.
It's best to add baseball dirt in the fall or after your season ends. Baseball dirt is more available in the fall than in the spring. The off season will allow the dirt to settle without players adding traffic while the dirt is loose. Unfortunately this task often gets forgotten till a few days before everyone starts to play. Dirt suppliers generally will add top soil to the baseball mix in the spring, which is not desired.
We are currently using a mix of 65% sand, 35% clay; that we call Frazier's Select Baseball Dirt. We've applied this infield mix at over 50 baseball, and softball fields. We are very pleased with the results. It's firm enough to make for good hops, but not extremely hard in the dry summer months. We will sell this product nationwide, but it is really only cost effective in Ohio, Michigan, Indiana, Kentucky, Tennessee, Georgia, and New York. Shipping rates beyond these states drive up the cost. As always, it's important to buy local.
Many other infield mixes are used. I've seen mixes of 60% clay, 40% sand which works, but it requires a great deal of care. High clay mixes need to be watered before the game starts, and sometimes watered again during the game. This is impractical for lower levels of play.
It's important to get dirt that is free of rocks or other contaminants. Do not use top soil for baseball dirt; it does not work.
We spread and level our dirt with a Kubota tractor and a Harley power rake. The power rake allows us to spread the dirt evenly without creating bumps in the field. I've seen it done other ways: Granville High School used a soil spreader, and it came out pretty well, but we still had to level it before the season started.
After we spread the dirt we check it with a laser level to make sure it's flat with a very slight grade toward the best drainage points we can find. If possible don't let all the water from the field go to the same spot. Balance the water toward several drainage points, ideally drainage tiles.