My advice would be to take a few seconds to think about what you are going to say before you type it. Make sure it is really what you want to say. In online conversations (whether it be in e-mail/forums or in real time chatrooms) you can always get away with a delay before you answer and no one will notice. As you get more practise you will get quicker at saying what you actually want to say and not being too 'submissive'.

It really is all about practise and study. Study pacing and how to represent emotions in narrative prose (a lot of reading will help here...). For example, short, rapid statements have more urgency than longer sentences and most action scenes will start with short sentences and then build up into more description. Its all about how it 'sounds' in your head when you read it so try to imagine someone 'saying it' as you write it. A good writing tip (not useful for o/l chatrooms but useful to know for e-mail or simply writing stories) is to read it aloud in order to relate the writing style with the sound.

You may also want to record yourself speaking and transcribe it and look at your own voice patterns and how you may write that as 'dialogue'. This may give you clues as to how you should write when talking in chat.

Another option for your online sessions may be to invest in a VOIP phone. Most messenger programs now allow you to connect a microphone to them to commnuicate by voice. If you get a headset with a mike and headphones in it (like the ones call centre staff wear) you can walk around while you talk as well and have your hands free. Make it wireless for more freedom... buy the same for your sub so you can have them perform tasks while still in communication with you - even if you have them in a position where they cannot type...