When you reply to a tweet, reply as if you are making a new post. This way, your replies are more meaningful to those who are seeing it for the first time -- which is just about everybody.

For example, consider the tweet:

"I think fudge brownies look like dog poo."

You might be tempted to respond:

"But they taste great."

Now if that tweet comes across my timeline, it meaningless and uninteresting. I could of course see what it is in reply to, but chances are I won't bother. Now, if you replies by saying:

"Fudge brownies may look like dog poo, but they taste great!"

Now that is an attention-grabbing tweet that would at least add a smile to my face. Plus, it would come up in the searches for the key words "Fudge brownies".

Reply with the other readers in mind, not just the one you are replying to.

Bounus: Spice up your posts with humor. Everyone loves humor! Say instead:

"Fudge brownies may look like dog poo, but they taste nothing like it... not that I would know. Just saying."