By being careful to keep a budget.
Two budgets really - one for fixed monthly outgoings like rent/mortgage, rates, utilities etc.
The other one for daily outgoings. Keep a record of every single penny you spend for a month.
Then you will see where the money goes. You will be shocked, I guarantee it.
Have a close look at your income against your expenditure. If you possibly can, put a fixed amount into savings every month.
Look at your bills - is it cheaper to change provider? If you have debts on credit cards, can you change to a lower rate?
Same with car insurance, phone provider - everything.
Boring and a grind, I know, but it will pay off.
I recommend this site to everyone I know:
http://www.moneysavingexpert.com/index.phtml
There is a budget planner there too.