Supports!!!
The only way you can convey a message is through supports & convoys.

The key point is to check all the orders, through the logs (link at foot of page) or large map (shows all orders). Particularly for Austria/Italy, if one is sending support-holds they won't show up on the map because the other keeps breaking them by moving & attacking.

Once you've got an alliance, if you want to support someone in somewhere, expect to repeat the support. ie, if you support him in in Spring, the odds are he won't know. However, if you then do it again in autumn he's more likely to order as suggested.
There are more tips, but those are the main ones

Cheers muchly!

Another thing is to unilaterally move troops away from a shared border. If they see that you trust them, they're more willing to trust you, and only bad Diplomacy players stab for a single SC (Unless it's the 18th). Just make sure you don't leave yourself TOO vulnerable. Plus, if he reciprocates, you both get more troops to send elsewhere, which is win-win.

A little note to go with that: be careful who you're playing with. If you're in a low skill gunboat, luck may well be a major factor

Also if it is a PPSC gunboat, someone may be quite happy to roll over you even though it lets someone else win.

Oh ya, I'm willing to do that, Alderian, if it looks like I'm dead. There's just no reliable way to setup a coalition in gunboat, so why not?

Stukus: I've been held up by Coallitions before, and formed them.
They key is to make sure you play with other people who're used to gunboat, that the game is PPSC, and that no-one misses turns!

"They key is to make sure"..."that the game is PPSC"...

??? Did you mean WTA? Or do you see an advantage in forming Coallitions in PPSC? Seems backwards to me, but if it isn't I'd love to know, because knowing is half the battle. Go Joe!

It's not impossible for coalitions to pop up in gunboats. Prime example: http://webdiplomacy.net/board.php?gameID=20218#gamePanel

Yes sorry, I definitely went WTA.