Concentrate on finding things you two have in common and can talk about so you can use them on possible future dates. Also realise that you're not gonna get laid/anything else on MSN, so if you are interested in more than chatting with her in your free time, don't waste time online and try to take it up to the next level and meet her personally asap (of course not too soon, I wouldn't try anything until I'd be sure her IL is high enough to actually accept a face to face meeting).
Anyway, I spent (wasted?) about two years of my life sitting on ICQ for long periods of time talking to girls (one of which actually ended up as a gf for some 8 months) and think I pretty much mastered that.
It took me way too long to realise the ultimate aim is not to have girls addicted to TALKING TO YOU, but to YOU. Huge difference. That's why I suggest that you step it up asap.
As for the convos, ****y and funny works wonders. It doesn't matter what you talk about, just misinterpret things she says in a funny way, make fun of her, etc. Some girls will react well to what/how you write, some won't. I found out that it is usually better not to waste your time with the latter, but again, what I wanted at that time were fun convos online. Often the girls who seem boring online are very fun to be around in the real world, so as long as you manage to make your part of the convo appealing enough so she agrees to meet you, you will be fine.
One last thing I'm gonna mention before this post gets too long/confusing are smileys (mostly the happy ones - smile, laughter, wink, *jokingly* etc.). Some people will disagree with this, but I think they are essential for making good impression - if you use them right. They add emotions, which are normally very much missing from online (and other non-face to face) conversations. Check how much she uses them and use them a little more. If she uses none at all, send an ocassional one. If she sends one after every sentence, match her.
Hmm, thats about it. But the most important thing is that you don't spend too much time talking to her online but rather try to meet her and only impress her once you do.