Two things:
1. When you have a natural flow going, follow your flow (follow your gut feeling)
2. if you are running out of words or things to say, try this technique -- read over the last sentence that your romantic interest said and expand on that thought. Maybe even tell a short story.
Example of technique:
him/her: I was at a mall yesterday.
(the keyword is mall)
you: oh yes, I like malls. Last time I was at the mall, I walked around everywhere and got a smoothie at the ice-cream place.
him/her: I ate dirt and was bored all day. boring boring boring blah blah
(the keyword is dirt - it's the most interesting one here)
you: I typically don't like dirt. Last time I got dirty was in 2006 and I still remember my mom beating the cr*p out of me for what I've done.
(now him/her will want to ask you about what happened)
etc. When you got a hang of it you can add nuances, like ask questions at the end, or leave out most interesting things out that will prompt your interest to ask about the part you left out, etc.
In the end.... don't be afraid to end the conversation when you feel the flow has ran out. In professional acting/entertainment/magic performances, one of the rules of performing for an audience is to "Leave them wanting more". It is better to leave your audience intrigued and interested and wanting more, rather than leave your audience bored, overstaying your welcome, or dragging things out too long. So when you feel that most of the fun has been had, don't be afraid to bow out. It is easy to do using text. Thousands of excuses are at your fingertips. "I gotta go eat", "someone just came over I gotta go", "my mom is calling me to clean my room, take dog to the vet", etc
