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The method using two consecutive top-ups is inaccurate for a variety of reasons and hardly representative of a particular driving pattern or period. Here's a more accurate and flexible method.

[edit] Steps

Top up the tank and note the odometer reading, d0 (mi or km).
Refuel as much and as many times as you want, anywhere you want, taking note of amount M (money) and fuel price P (money per gal or L) for each refueling (receipts may help if they contain the data).
When you want to end the campaign, top up again and note the data as in previous step AND the odometer reading dn (mi or km).
Calculate the quantities for each refueling except the first one using the formula: q = M/P
Sum the quantities: Q = q1 + q2 +...+ qn, which gives you the total of gal or L bought.
Subtract the initial top-up odometer reading from the final one, which will give the distance traveled with Q gal or L: D = dn - d0.
divide the two: FE = D/Q and that's your fuel economy, in mpg or km/L, for a longer period, therefore more accurate.
In this manner YOU decide when to start and when to stop the measurement campaign; the longer it lasts, the more accurate and significant the result.

[edit] Tips

Why don't we use fuel quantity directly?
The reasons behind using money and price to calculate quantities is that for legal reasons both values are well-defined and document two hard facts: the expenditure and selling price. The rest (displayed quantity) is less accurate. Besides, reading fractional quantities from some (especially analog) displays is far from exact science.
Why do we consider more refuelings?
First, because you want to find out your economy for a particular trip, or type of road, or season, or mix which does not necessarily coincide with two consecutive top-ups. Second, calculating anything based on a series of intermediate measurements is less accurate as it sums small measurement errors which don't cancel each other.
A car's fuel efficiency constantly changes depending on weather, routes, temperature, and driving habits. Only take your last odometer reading and subtract your first one and divide by the amount of gallons you used to refill your car. Your first odometer reading should be taken when your tank is completely full.