First, when choosing hitch jacks, it's important to know your trailer and its tongue dimensions. For example, you will see jacks that say "fits tongues up to 3" x 5." Some hitch jacks bolt onto the tongue itself; others require drilling, and some do not come with installation equipment.
Next, know how much you'll be carrying. Your jack needs to be strong enough to lift what you'll be towing. Fortunately, there are a wide variety of jacks out there to suit your hitch, from ones that can pump up an 800-pound mini-trailer with a motorcycle on the back to heavy-duty jacks that can support 5,000 pounds or more.
You should also note the ground clearance of the tongue. The jack won't do any good if it can't reach the pavement, so jacks come in different lengths to suit different trailers. Some jacks come with wheel casters on the bottoms. These small wheels aren't meant to be left on while you're driving, they do make storing the trailer in your garage a little easier. If you're towing a boat, get a hitch jack that won't corrode or rust with exposure to water.
Finally, you may want to consider purchasing a power jack. Rather than having to manually crank the trailer up and down, which can be time consuming and physically difficult, these jacks plug into the vehicle's battery and raise and lower themselves. These are often more expensive than manual towing accessories.
In the next section, we'll learn how to stay safe when using a hitch jack.