Also: do NOT sign up if you aren't going to show up!
This is very important. It's very hard for the clans managing the roster for a fight to make sure everyone who has been accepted for the fight is actually there, so make sure you don't sign up but forget to check before the battle if you got in.
When you apply for a battle, always check if you get accepted. If you do, be sure to check in with the clan that hired you on time on their teamspeak, else you might still get removed.
Oh and if you get accepted and then don't show for a battle, you might very well be blacklisted. So make sure you don't sign up unless you are able to check if you are accepted an hour or half an hour before the battle and make sure the clan that hires you knows you'll be present.
If you are not accepted, feel free to join the clan's teamspeak and offer your services one more time and ask to get in. It's best done by chat message to the person managing the roster. Or use voice and clearly state your name and ask to be let in.
Also, try and keep quiet as a hired fighter on strategus battles and let their commanders command the battle uninterrupted. Be careful not to give any orders. But feel free to offer accurate, precise information (like: i see 5 cavalry approaching on the right flank behind that hill).
The thing that's most important to commanders, i guess, is reliability (showing up on time, being present on teamspeak), the ability to follow orders and be patient (very important in strategus, prepare for long fights with lots of shieldwalling, camping and such) and the ability to know when to speak and when to shut up. If you want to offer a suggestion, make sure it sounds like that: "commander, i suggest we flank their right and take the flag, nobody is guarding it at the moment" and allow them to make the call.