You could always work longer hours in your present part-time
job, although the chances are that you're already working as many
hours as you reasonably can without it starting to have a serious
effect on your studies.
You may wish to try some of the following techniques which may
make that part-time income go a little bit further:
Work on Friday or Saturday nights
By working a shift at the weekend, you then become less likely
to need to go out yourself. Having to be at work is always a
much better excuse to your friends than claiming that you are
going to try and study on a Saturday night, and drinks are usually
much cheaper during the week anyway!
Work in a bar or restaurant
Depending on whether food or drink seems to be your biggest
budget eater, by working in a catering establishment, you may
find that a certain amount of free food or drink is included
as part of your shift allowance. Pizza restaurants can be particularly
good, as you may be able to stock up using the salad bar. An
additional bonus of working in a restaurant, especially in popular
tourist locations such as Edinburgh, Bath, Oxford or London,
is that your paltry part-time income may be supplemented by
tips from the occasional generous punter.