COMMANDING THE OFFENSE
The truth is that most of your allies know how to use a dispenser, most know they should stand still when you're trying to give them armor, and so on. But they don't do these things because they aren't in the right mood and don't have the right goal. What you really want to do is to give your team morale and develop raport with your teammates.
First, the things to avoid. Don't use vague, stock, useless messages and repeat them five times in a row. If your messages are vague ("Incoming Enemy by LAND!!!"), people won't know what you are talking about and just think you're stupid. If your messages are stock (Assemble troops in central battlemes and attack main enemy fort.), people will just ignore them like they ignore junk mail. If you repeat the message and make tons of annoying beeps, then people will ignore what you're saying just out of spite and will like you less: only a schmuck would react to such an elicitation anyway. So, keep it short, use abbreviations, and make typos to add authenticity.
If someone on your team makes a comment or a suggestion, and you can tell that no one is going to respond, you should give, at least, a minimal response. There's nothing worse that saying something to your team that could use a response and getting nothing back. By answering, you raise your ally's sense of teamness instead of letting it get lowered. Since you are the one who responded, your raport with that person will go up, and that person will be more likely to listen to you later on. Also, you establish yourself as the leader of the group because you just judged whether a suggestion was right or wrong, a comment good or bad.
Another psychological method or trick is to give little thanks for what people do for you. If someone upgrades your sentry for you, say "thx" or "ty". Don't say "thank you", it's too oppressive, and actually causes people to help less since they don't want things to get too chumy. Communicating at the right level of chumyness is important.
Thank any action that is teamlike. If you're fighting some guy and a sniper kills him for you, thank the sniper. The sniper probably just wanted a frag and could care less about you. But with the simple act of thanking him, he will beleive he actually did do it for you! And, he'll start to try to help other allies that are in trouble (cognitive dissonance and reenactment).
The one time to go buck wild is when your team scores. Not only should you congratulate the person who got the flag, but you should say things like "boya!" and "finally we got them", since that gives the impression that you are glad the team got the flag and not just the flag carrier. Every once in a while, echo the team score back to your team; say, "30-0, we're kickin ass" or "10-0, good now we're in the lead". Basically, just point out that the two teams have a score and we want our team to score higher at the end of the game. This reminds your teammates not to just look at the number next to their own name.
Here's another example: "fr disp at se enemy gas area". Another: "en sg at 9oclock in en yard". The "9oclock" is essential since there are almost always sentries in the enemy yard anyway. So, otherwise your comment would be basically useless.