how are you applying for jobs? Online or in person?*
For me, when I was younger - Online applications were a waste of time for anything part-time retail based. Best thing I ever did was just to approach it the old fashioned way - target business that do not go the online-application form route (small buisness, pubs, bars, resteraunts) and just go and give them a CV and talk (definetly talk, dont just give them a CV and then run) to them in person.*
Most are going to reject you, but if your persistent, have a good attitude, and are not a complete idiot - then eventually you will get something.
Also, go for jobs that are used to employing students, student bars/cafes etc, close to the uni. Many jobs will reject you because they know you will disapear for 1/3rd of the year to go home (sometimes at their peak times, in the summer etc) - so first target those buisness that cater to students, and are used to employing students. if this does not work, get very good at quickly explaining how you will not be going away from university in the summer/winter/spring holidays.
Check the uni job boards frequently, and look for opertunities within the university itself. from my experiance open days are a nice source of bonus cash.. I used to do 3-4 a year, for around 40 quid each. Its not much, but its better then nothing.*