So many people spell it incorrectly that I'm starting to worry that "sike" will become the official spelling. Language evolves, but I don't want it to.
If you think "of" and "have" (or it's contraction) sound alike, you're part of the problem. "Of" has a softer finish more like "ovf" than "ov". The transition from 'o' to 'f' starts hard with a 'v' but finishes with a soft 'f'.