If you're going to be working on your mower, you'll eventually need something to clamp a hose with. You can buy little twist clamps that usually come in a set of two, but for a couple bucks more you can get a hose clamp tool -- the Lisle Hose Pincher, model 23000.
The Lisle Hose Pincher looks just like a pair of scissors. It works great for clamping fuel lines and other rubber hose. Just open the grip, place it around the hose, and squeeze. The ability to use it one-handed is very helpful. The tip is angled for both small hoses and large hoses. It locks like a pair of hemostats, and you unlock it by pulling the handles away from each other. Sometimes you need two hands to unlock it though, if it's gripped tight.
The only downside to this hose clamper is that it's kind of big, so it may not work for hoses in tight places. I had to wriggle it a bit to get my mower's fuel line, but it eventually fit. For a small space, you might be better off with a smaller clamp, but if this one will fit it works great. You can just lock it and leave it attached to the hose while you drain the fuel tank, remove the carburetor, or do whatever you need to do. And the tool is plastic, so it won't cut into the rubber hose while it clamps.
You can get the Lisle Hose Pincher on Amazon.com for $9.95, cheaper than most hose clamp pliers and only a buck or two more than most small screw clamp pairs.