Well it looks like there will only be two remaining space shuttle missions, so if you are not an astronaut by now, you probably wont get onto these flights!
No worry, since through the Nasa Face in Space program, "NASA wants to put a picture of you on one of the two remaining space shuttle missions and launch it into orbit."
Apparently entry into space is as simple as uploading your image/name and it will be flown on one of the last two shuttle missions. After your image has "flown", you can "print your Flight Certificate - a commemorative certificate signed by the Mission Commander. You can also check on mission status, view mission photographs, link to various NASA educational resources and follow the commander and crew on Twitter or Facebook."