Yeah, IRCs are a thing of the past, but cold hard cash still gets results, for about the same price if not less. The most I can suggest in letter writing is to try and be sincere. It can be hard to be original in a letter after writing 20-30 requests. They all start to sound the same. But, I find it's helpful to review the actors credits on IMDB, and see if they were involved in anything else you liked. That can give you a little something more to talk about. I would sometimes ask the actor to help clarify which character they were in a scene...(like troopers, pilots, or guards). Sometimes they can speak specifically about it, sometimes they can't. I found it was a good conversation point, that way I could place there autograph with a photo later, in the case of background characters where it is uncertain if they are even in the picture you send. I would also sometimes ask them to suggest films they have been in to watch, if I wanted to see more of their work, or ask if they have any upcoming projects they might be appearing in.
I wouldn't try and force a conversation if you just can't come up with anything. That's perfectly ok. However, I agree with SWfan. You can sometimes get some really cool responses...even whole letters in response...sometimes handwritten.