Have just done ours, so in full money-anorak mode:
(a) Get a notebook and write down everything you spend for a month.
(b) Do a spreadsheet with fixed expenditures (childcare/rent/bills/cleaner/travel etc).
Once you know where your discretionary spending under (a) is actually going, you can decide target amounts and fit it into your budget under (b).
The two best websites for this kind of thing are The Motley Fool (you need to register to read the discussion boards, where most of the good tips are) and Money saving expert , which will mail you a weekly newsletter with current tips, but has a lot of useful info on the site and the chat forums as well.
The sections of the boards on dealing with debts are particularly enlightening in terms of seeing other people put up lists of their expenditure and having it radically pruned by other contributors. Once you've worked out the amount that you need to spend each week, it can help to withdraw exactly that amount in cash on a fixed day each week, and make it last through the week.
Otherwise, just the obvious:
Check your phone/broadband/TV package -- there's a lot of extra competition atm. Telewest reduced our package by nearly half just because I asked;
Think about changing energy suppliers in a month or so once the current round of price changes has stabilised;
Check the timer settings for your hot water/heating. I've just shaved 2 hours a day off the hot water programme, and we still seem to have plenty.
Meal plan, and do food shopping in Lidl;
Don't buy kids' clothes new;
Plan for christmas and birthday presents well in advance, and stock up in TK Maxx etc so you don't get caught out by unexpected birthday parties. Ditto wrapping paper and birthday cards;
Once you get in the swing of it, just not spending money works pretty well. I find that while it's easy to persuade myself that we need particular things, if we just don't buy them, we seem to muddle along perfectly well without.