This series of drills focus on targeted weight training using a weighted bat or your first baseman's bat (she is generally the biggest player on the team, so she should swing the heaviest bat).
Have the hitter take a weighted bat or the heaviest one you have on the team and hold it in both her hands with the barrel pointing straight up toward the sky.
Now using only the wrists, have her lower the barrel of the bat to her left to about waist high, then to her right and back to her left, etc.
Do about 5-25 of these depending on the age of your players.
Take the heavy bat again and have the hitter hold it directly over her head with the barrel pointing up toward the sky.
Now, in a chopping motion, have her bring the barrel of the bat first backward until it is pointing down at the ground behind her and the bat has touched her back lightly.
Now bring the barrel back up to the start position and continue forward and downward so that it lightly touched the ground in front of her (this motion is like chopping wood).
Have her raise the bat slowly to about waist high and have her bring it slowly to the ground again, then back up to the start position.
This constitutes one rep. Have the player at this station do 5-20 reps depending on the age of your players.
This drill again uses a heavy bat. Have the hitter take a normal batting stance and have her swing into the contact zone.
As she reaches the far limits of the contact zone, have her use her wrists to wiggle the barrel of the bat back and forth as she counts the wiggles to five. Then bring the bat back to the normal batting position.
This is one rep. Have the hitter do 5-20 reps depending on the age of your players.
The final drill again uses the heavy bat. Have the hitter hold the bat out in front of her with the barrel pointing straight away from her body. Her arms should be straight out with maybe just a slight break in her elbows (I mean very slight break).
Now the hitter makes small circles with the barrel of the bat by moving her wrists in a kind of circle. First, have the hitters do 5-15 circles clockwise and then the same number of circles counter clockwise. This will strengthen the wrists and make then quicker.
I use these weight-training drills in practice at stations. Most of the time, you may have a couple of players waiting to get into the batting cage or go on live pitching. Since I feel "standing around time" is wasted time, I have these players do these drills instead of talking to their team-mates about class yesterday (yes I do know what they are really talking about and it is not softball or class).
With all of these drills, try this out in the backyard or with your assistant coaches BEFORE going the field so that you can instruct the players in the proper mechanics of the drill.