Kickball is about having fun, why not extend that to the boring t-shirt you get for each season and spruce that thing up. A lot of these modifications are used to sexify a plain t-shirt but some of these will give you some more airflow if you are playing in the summer and would like to sweat a bit less.
These all pretty much just require a pair of scissors so really no excuse to show up to your next kickball game in that boring ass shirt they handed out to you at the start of the season. You will be making permanent changes to your t-shirt so make sure you have a spare and/or practice a couple times on your non official team shirt first. Keep in mind you can combine a couple of techniques to come up with a truly unique look.
Back Weave T-Shirt
Awesome blog post from my GF over at MakerWoman.com on how to open up and create a variety of very interesting weave patterns into your shirt. This is a bit technical and can be tricky to do on cheaper material, so make sure you practice a couple of times before doing this to your team shirt. For an extremely detailed guide, check out her post on making a t-shirt weave.
For a video of making a similar style t-shirt weave that will open up the whole back of the shirt (you will need at least 1 or 2 sized bigger shirt for this), check out below.
The tshirts you get for kickball are going to be standard mass produced one-size-fits-all. If you want to get something that will actually fit to you, here is how you turn that boring shirt into a nice fitted tee. This will only require a pair of scissors and some careful measuring. You may want to go up one size in t-shirts to give yourself some more room for making these modifications.
This is one of the simpler modifications you can make (5 minutes and a pair of scissors) and is one that works for a guy or girl. This change will give you a nice hippie look for the girls or a hulked out look for the guys. You can also do the same types of cuts to the bottom if you do not mind your stomach showing. For a how to make this modification, check out this post.
Side Weave T-Shirt
Want to add some side flair to your shirt now? This one is a little harder to get right but it will look very impressive if done right. Head over to Instructables for the how to do a side weave.
Off Shoulder T-Shirt
Another great look for the kickball field to make yourself stand out. This one will require a bit more than just a pair of scissors so I hope you have a little bit of sewing skills or a friend that does. Go here for a detailed look on how to create an off the shoulder t-shirt.