I have read all of the novels by Agatha Christie and Dorothy Sayers, and now wonder what I will read next. Can anyone recommend additional authors to me that are similar to Christie and Sayers? Thanks in advance for your help!
Some authors that remind me somewhat of Christie:
Ngaio Marsh
Margery Allingham (VERY hit or miss for me)
Patricia Wentworth
John Dickson Carr
Nicholas Blake
Most of these can be read in any order.
The only author who has ever really reminded me of Sayers is Sally Wright, starting with Publish & Perish.
If you can say what you find appealing about Christie and Sayers, I might be able to suggest other authors, too.
This would be my suggestion for somebody jonesing for Sayers and Christie. If you have no prejudice against American detective fiction, I’d suggest giving Rex Stout a try, and maybe Ellery Queen (probably nothing after 1960 for Queen).
Kinky Friedman 
You are a bad, bad, man.
Anything I could say is probably already covered in these threads:
Is there anything I can read that will REALLY fill in for Peter Wimsey?
J.A. Jance. I like the Joanna Brady mysteries. The J.P. Beaumont novels are also good.