Know your aircraft !
I agree with almost all that IO540 has to say on the subject, however if you know the aircraft well you don't have to have any more than an EGT indicator to get accurate fuel flow numbers.
On both the Robins that I have owned (same engine as the PA28) I could get the fuel flow numbers to the nearest litre, I fly (by UK GA standards) long flights direct to northern Spain and the far south of France from Oxfordshire and have never had a fuel management problem.
To achive this I make sure that the power and mixture are accuratly set and at the end of a flight check that the fuel uplift matches the plog fuel burn (to monitor engine performance).
As for fuel planning I use the basic commercial reserves as outlined above however on the longer flights I use the en-route alternate system. this cuts the contingency fuel down to 5% overhead the destination airfield and gives about 30 Nm more range but requires carefull fuel monitering
at each waypoint along the route.
The biggest problem with the average PPL holder is poor education when it comes to fuel management, most clubs treat the mixture lever as an "on-off switch". I have found when doing recency checks for a local flying club that most pilots don't even know how to set 75 or 60% power and this is essental for accurate fuel flow at these are the numbers in the flight manual with the performance graphs.
Can someone please tell me how most new PPL holders have got a licence and never used the mixture lever correctly?