As voltage goes up, resolution goes down. You don't want to look at the entire 24 volt range. Just the last 5 to 7 volts will do. A 24 volt supply running at 18 volts is failed, so you don't care about the 0-18 volt portion.
Use the 10 volt input mode. Figure a maximum of 28 volts, and low end cutoff of 18 volts.
Put a 18 volt Zener diode in series with the line. 1N4112. Mouser, Digikey, etc. Use a 1 uF, 35 volt Tantulum cap across the input to filter the noise from the Zener. If you're stuck going to Radio Shack, 18 volt may be hard to find. You can use two 9.1 volt Zeners in series, or any other combination.
That's a $2.00 solution, and nothing gets hot.
To make it safer, add a 250ma slow blow fuse in series, and a 10 volt, 1 watt Zener across the input. This will protect your card from overvoltage. It probably has an internal Zener, but I doubt that it has a 1 watt rating.
The resistor divider solution sounds easier at first, but heat is a problem. The Zener circuit will run much cooler. Radio Shack (and others) have small prototypings PCB's, and matching project boxes.
The 1N4112 is a 1/4 watt Zener. You could also use 1/2 or 1 watt types.
If the A/D input impediance is high, the Zener might 'sawtooth' in and out of conduction. This will appear as a few millivolts of noise. Add a 10K, 1/4 watt across the input to lower the impediance.
The series string is: +24 - 1N4112 - fuse - +A/D input.
Parallel circuit is cap, 10V Zener (1N4740A), and 10K across A/D input.
Can someone post a schematic for me?