Go to: User Cp at the top left and select Edit Options. You can control the number of posts displayed. It also gives options for subscribing to posts although they are not quite as flexible as you are wanting.
When you reply, use the "Post Reply" button rather than the Quick Reply box. Under the "Reply to Thread" box will be "Additional Options", if you scroll down some you will notice "Thread Subscription" that allows you to subscribe to individual threads as you post.
The Quick Reply box doesn't allow this feature, but if you want to use it you can set your default to subscribe to every thread automatically and then edit your subscriptions before you log off the forum. When you click on your UserCP you will see your subscribed threads in the center menu.