I'm taking my USMLE step 1 June 1st and obviously the comlex a week later.
I've talked to tons of people and honestly what I've found is that there is no "good" way to do it except figure out for yourself what to do for YOU to perform best on both.
I have a classmate that's taking the USMLE the next day following two grueling days of COMLEX, I would be totally exhausted and not care about a single answer so I chose to go a week to give myself time to rest up. I'll be ready study wise and have a week to recover from any tired-ness (although I don't think that one day will compare to the two-day comlex physically)...
If you wait until rotations start finding a prometia (sp?) center can get complicated and scheduling even tougher as most places don't have saturday hours long enough to accomodate the USMLE on weekends.
I took the USMLE 10 days after the COMLEX last year and I felt it was a good amount of time in between, although I think a week in between is enough. You need to give yourself enough time to:
1. take at least one day off after COMLEX-dont disregard how burned out you are going to be after that test. It is 2 days of sheer hell and you will barely remember your name when you walk out after the second day. There is no way you will be able to study the next day let alone take another test
2. There is a lot of molecular biology and biochem on the USMLE that is not on COMLEX and you need to give yourself enough time to brush up on that stuff. I would suggest trying to read first aid at least one more time, do questions and do biochem/molecular in between the two exams
Good Luck-I know it sucks to take both but I'm really glad I did it!