Ive not worked in Ibiza but I have worked abroad in resorts similar and my Gf has worked in Ibiza
If you are prepared to work hard for crap money then u'll always have a job.
If you are going over then go in may or even earlier although things don't start properly until late june the best jobs go early and you might get something that pays ok with accomodation thrown in too.
A job with accomodation is going to be your best bet as it will save u loads of cash on rent.
Be friendly and try and learn pigeon spanish. Most bar owners are spanish and if u can make an effort to be able to talk to them in there language they warm to you a lot more.
If u can speak full spanish then you will find things so much easier and even better jobs.
If you are really outgoing and have the gift of the gab then there is money to be made in PR ing but only if you work like a maniac.
The best Pr jobs are with the Big clubs who will often give you commision on people entering the club with your flyer.
or u can work the bars but u will find that won't be the most liked breed on the island and you will have to work in san an as they don't have bar pr's in ibiza town there are a few places in figuretas but not many.
Also try and network and get to know as many people as possible not only will this help with jobs but other things like getting free drinks and free entry into clubs.
Try and become good friends with a local as they are not only good company ( spanish are friendly laid back people ) but they also know plenty of people ( bar owners e.t.c. )
that should help a bit