When you've just said goodbye.
Resist the temptation to pepper your date with "that was great" messages before you've barely paid the bill on your dinner together. Let a little time pass. When you do text later, make sure it has a purpose (no pointless "what's up" messages!) -- by wishing her luck in that meeting she mentioned, or asking about his sick dog.
When you should call.
You'll get carpal tunnel syndrome having lengthy conversations in place of simply picking up the phone -- the same phone you have in your very hands. Texting is great for simple bursts of information ("see you there at 7"), but if it's something that will require more than three exchanges to hash out, just dial.
When you're plastered.
You knew this one right? Flirting via text can be sexy, unless your "tempting" message is "r u coming over?" and you send it from the bottom of a pitcher of margaritas. Even if your date can laugh with you about it later, it will still likely come across as too forward in the early days of your relationship.
When it could be misinterpreted.
Texting doesn't communicate nuance well, so avoid using it for strong emotions, positive or negative. "Where have you been?!?" or "Why haven't you called me?!?" might make you sound angrier (and crazier!) than you really are. Even sarcasm and jokes can come off as meaner than intended. Even with a smiley face and LOL, something that seemed hilarious on your screen might be offensive on your date's.
When it's the middle of the night.
If you wouldn't call at 4am, don't text then either. Be sensitive to what the person might be doing; don't assume someone is up early on a Saturday morning or late on a Friday night. Even if their phone is off and doesn't disturb them, what message do you think you're sending when they see that late-night time stamp?
When you've just texted.
It's easy to fall into the habit of texting as a regular form of communication. But too much texting discourages "real" communication via phone conversations and actual dates. Don't use your mobile to give your date a daily play-by-play of your life, and always try to receive about as many (or more) texts than you're sending out.