I entered the USA on a three month visa waiver program from Australia. I am about to return to Australia and would like to come back again to the USA. How long do I have to wait until I can re-enter the USA for another 3 months?
I think you can enter again the day after you left. I remember entering again about 100 days after I entered the first time (so not long after the end of the 90 days, but a while after I left the country) and I have friends who entered a lot of times and on a regular basis so they probably had maybe 2 weeks between the time they left US (and gave their I-94 back) and they entered again (filling a new I-94). We were all European and part of the Visa waiver program.
On an official website no other condition than having a valid passport (and the ESTA procedure if you arrive by plane) is required.
Be careful, though, to do the ESTA procedure if you arrive by plane and to give I94 back when you leave the US (if you got it when entering - it is the green paper with these weird questions)