My guess would be that you are not allocating enough money for all your expenses therefore running over budget.
We had $800 a month for food... in reality we are spending at least $1000 so I've tweaked ours to increase the amount for food and I was also underestimating petrol costs.
If you are tracking daily expenditure then you should be able to see where you are going over.
When we go on holidays I figure out how many days we are away, how much on food, spending, accommodation so a total per day, then we write down everything we spend and make sure its on or under that amount. A day or two you might go over and then you know you need to go a day or two under to get on track again. This worked really well for 2 long trips we did (8-10 weeks). I wish I could do same for when we are at home, we never went over budget on holiday.
Also make sure you have included or have on the radar ALL bills (rego, insurance, health insurance, petrol, gifts, clothing, maintenance/repairs). We do our budget a year in advance and know when all the bills will hit (going by past bills due date and amounts).
Even when you are vigilant its still so easy to go over... because life is V. expensive!