As a full-time student I have no regular income. Living on my savings, however, is definitively not the most productive, so I try to grab any opportunity from which I can support myself.
The most logical way is probably to work. If you have a supportive supervisor you can probably get some jobs from within the university itself. In my case I have so far worked on a project which was being carried out in the centre I am based in. This meant that besides putting some of the skills I have gained during my research into practice, I also gained some relevant experience within a research project besides my own.
However, there are other ways in which a student can get some money to support themselves, which may not always be the most logical.
So far the most lucrative possibilities have probably been applying for posts offering fixed grants. For some reason I always manage to come way under budget for these events, which means that besides gaining quite a lot academically, I also manage to support myself for that bit longer as a student.
Another way of making some money is then from participating in focus groups and/or research experiments. These opportunities mean that for giving up some of your time you are either paid in cash or vouchers. If I am paid in vouchers I normally try to choose vouchers from places I know I need something, so that the money can go towards something I really need.
I have been lucky to have managed to save some money during my previous degree from working and stipends. However, these small amounts have definitely built up to mean that I feel less guilty of treating myself every so often.