What can I do now about the cracks in my joint compound?

I'm drywalling...I applied the joint compound too thickly and I still need more coats. The problem is that the old drywall expanded over time and the new pieces that I had to put in wouln't fit flush with the old. So what now? Do I wait for it to fully dry, then sand it, then just add my remaining layers of joint compound in thin coats? Should I put more tape up first?