I use a spreadsheet.
It's simple - just break your income/costs into fixed and non-fixed each month. That them provides the basic cashflow. I then repeat each month for a year and that gives me a reasonable picture.
I break a utility bill into fixed and non-fixed, so for example a virgin monthly contract cost would be fixed but I would maintain a track of the additional cost of calls to give an average and place that value into the non-fixed for each month going forward.
There are things such as car servicing etc that I'd put as non-fixed but I would estimate, say £500, for the service and add it to a future month when it's due etc. I can then see if I need to save before I get to that month.
I then take the actual cashflow and update each month as I go and update any forecast numbers if the numbers are not reflecting reality.
Additionally you should look at the outgoings and cost cut if required (find a better deal etc). By considering a year rather than each month as it comes you tend to think about this sort of thing as you see the savings.
So if you have a row for each type of expense, such as mobile phone etc, you can make a pie chart of your costs very easily which helps to show the costs over a year for example.
All that text is confusing,
here's an example layout as PDF
What's available is basically what you have available if you need to spend.
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