Too often I see new players waste all of their paint in the first 15 minutes of a 30 minute battle. Get yourself a good durable watch and know how much time you have left in the game. I like the Timex Ironman brand, mine has taken tons of hits over the years and still works great.
Guns with LCD panels from Planet Eclipse
all have game timers built in to the grip. Set the timer to count down so you know when the final battle is getting close to ending.
A lot of paintball players don't carry a watch so don't be afraid to call out the time left every so often. It should (hopefully) help to give them an idea of how much time is left and how much paint they need to keep in reserves.
Remember even if you run out of paint or air in the final battle you can still help your side. You can call out positions, be a target to distract the other team, score points by flipping flags or other tasks. Even if you are just on the field it will help to make your team look stronger to the other side. Don't run around shooting just air because quickly that will let the other team know you are out of paint.