if she has to scroll, the message is too long.
if you dont have a keyboard, be brief.
if it's not worth writing out, don't write it at all.
isolate the technology, in other words, 'wow these txt msgs are really sweet im glad i got the kewl new rotomoler 3027b with boring tech advanced beta 2' is not something that needs to be included in a message
remember that you are talking to someone, that's key
don't be too confusing cuz you know what eats a large magnitude of butt? having to 'find' the right word in a text message.
and the most important of all: IT'S A LOT FASTER TO CALL SOMEONE, SO ONLY TEXT IF YOU MUST. don't be a pusstachio
a lot of this stuff can be applied to say, 'talking outloud' or IMing...but there's a reason for that. good day chap.