We have also been keeping a daily record of expenditure and kilometres travelled. As previously mentioned, when we are arriving in larger towns we fall into the usual shopping / holidaying mentality, and tend to spend more. With the kids we like to take them places and on tours which can be expensive, however this is all part of the adventure, but we do make use of free camping alot.
Lazing on the beach or in one of the waterfalls is a great way maintain the budget and keep the kids happy. Its just a shame that Julie and I have very short attention spans and tend to move on quite quickly, which pushes up the diesel costs...