I've been self-employed for a few months now but before that, my day job earnings weren't enough to sustain us without my self-employment income, so I had to learn to budget early.
The most important thing to do is work out your budget and then start building a 'cushion' which you'll be able to dip into in the low-paying weeks/months.
Work out all your outgoings (for non-regular bills, like insurance, car repairs etc, estimate how much they would be yearly and put a 12th of that money away every month. I also do this for Christmas, birthdays, holidays and business costs). Try and trim every expense down to the minimum as Fluffycloudland77.
Once you've worked out what you have to earn to cover the essentials, start to put away money (when you have a good week) in a 'cushion' fund. This will gradually build up, so when you have a week/month where you haven't earned enough to cover your essentials, you can dip into this to make up the difference.
If you're being really efficient, you could build your cushion to cover 3 months to 6 months of essential costs, that would really bring peace of mind (although might take a long time to achieve).