Foreplay includes everything that happens before the actual shagging takes place, so keep that in mind before you send a picture of your dick. To some degree, sexting is the Internet's answer to no one ever having enough time to do anything with anyone else IRL, and this, sadly, means foreplay. Use your sexts to tease and cajole your partner into asking for more.
The number one rule when it comes to dick pics is this: Do not send a dick pic unless one has been solicited. If you send one without knowing for sure that the recipient wants it, you run the risk of being rejected. If you are dying to send one, ask your sexting partner if it would be cool before hitting "send." Use your words and send pics of other body parts. If you play your cards right, you'll get the green light on a dick pic in no time.