Dream Machine

[Burpzilla]

Tools --> Internet Options --> Content --> Autocomplete --> User names and passwords on forms / Prompt me to save passwords

That way you can regularly dump your temp files & cookies, but when you come back to a sign-in form you can just double-click on the sign-in box and your password will be entered automatically.

You may want to clear passwords first at that same location to get rid of any standing info.

Great feature.

PS: That still deletes the cookies at the sites you frequent (which was your question), but it does give you a quick double-click method of getting in without retyping your user name and password.