If your active duty stay on top of your fitness. Look at what typical agencies test you on (mine uses Cooper fitness standards so you will run 1.5 miles, sprint 200 yds, leg press and bench press % of body weight and situps based on age groupings)
If you have any college credits, make sure you get your transcripts information in order. Degrees same thing.
Pull your credit report and drivers history, if there are any mistakes now is the time to fix them.
If you have the opportunity, consider taking a spanish class. Use to be spanish was a plus in the mexico border states and Florida, but this has moved further north. You may not be a translator, but the ability to communicate simple sentences is a plus.
When I was in the military, I didn't own a suit. I bought one before I got out and it came in handy. Get a good conservative suit, shirt, tie and shoes now as its better to buy one now than when your pinching pennies after you separate from the military. When I went to my oral board, there were 33 of us that made the cut for the board that was the last step in the hiring sequence. 33people competing for 1 opening. 2/3 of us were in suits, the rest in professional casual attire and some in street clothes. Most agencies place value on appearance and image, show the board that you take it serious.
Try to familiarize yourself with the applicable laws if you can find a reasonable resource to do so, but it can seem like mumbo jumbo outside of a college class or academy setting where instruction, structured learning and discussion tie it together.
Lastly, and this is a long shot for most, but I completed my standards (through a local basic recruit course at a college) while I was still on active duty, so when I got out I was already state certified since I had passed the state exam. I got hired on with an agency and had to complete FTO and probaton. Some agencies may send you to an academy, but having your certification takes alot off the agency and the new hire.