How can I get my own apps onto my jailbroken iPhone?
This article really helped me.

I'm not sure if it's still that relevant though. I remember just setting my xCode to "Don't code sign" or something similar. I can't really check it right now, I'll get back to this. Make sure you have appsync installed on your iPhone though, that is a must.

After that you might be able to run the app on the iPhone, but you won't be able to debug it live.