Maybe you need to ask your ex something. Maybe he still has your stuff. Maybe his dad is ill and you want to check in. When you do send that first text, accept that you may not get a response – or at least not the response you want.
One-word texts. Don't just text "Hey" or "Yo." If you have something to say or ask, do so in your initial text. Get right to the point. Don't put your ex in an awkward spot of trying to interpret why you're contacting him/her.
Terms of endearment or flirty emoticons. You're no longer a couple. Avoid using pet names or cute phrases that should be reserved for a relationship. You cannot keep the cute stuff post-breakup.