If you aren't to bothered about travelling in style then the Disco Bus runs regularly and, as said above, is very cheap. I am 99% certain that for a lot of nights you can buy your tickets from places like Mambos and get a free bus over. Last time I did that was in about 1999 but I am sure it still happens. If you want to go over earlier and have a drink in Ibiza Town (which I would recommend) the day-time bus service is regular, cheap (about 2 Euro) and takes very little time to get there from San An.