Overalls, but also jumpsuits, jumpers and the like, usually have a way to make them fit around your waist, like a zipper or buttons in the side. If you have a suitable one, closing it prevents you from lowering it past your hips (or over your shoulders). That means that if you prevent that fastening from opening, the victim is stuck inside.

The table below discusses the different fastenings:

The size of the padlock is critical A padlock around button and through buttonhole The same padlock turned around to the inside	Standard button-and-hole The most standard waist fastening on overalls. You need to find a padlock that has a slightly smaller shackle than the button (see illustration), and lock it over the button stem while the button is in the hole. If the shackle might just be a bit big, you can also include the fabric of the buttonhole, like in the second picture. The extra advantage is that you can rotate the padlock and it disappears inside the overalls. See also Ariel in Bright Blue Denim Overalls photo 7. This method of course also works for straps that attach to the bib with a button. If there are buttons that don't need to open to undress, you can sew the buttonhole a bit smaller, so that the button doesn't fit through anymore. That's quite invisible. Jumpsuits and the like might have buttons that are too small for this. In that case you can sew an O- or D-ring next to the button, and lock the buttonhole to it. On the other hand, if the buttons on the jumpsuit are in the back, they will probably be very hard to undo for the wearer, and there'll be no need to lock them! Knowing this, you could also try to put on a front-buttoning jumpsuit backwards :)
Skortall with back zipper and locking buckles Detail of padlock through zipper pull Padlock through zipper pull, with padlock body hidden inside garment	Zipper Most common on jumpsuits, but there are also overalls with zipper at the side(s), front or even back (as the skortalls in the illustrations). 'Disadvantage' of overalls with zipper front or back is, that opening the zipper will most certainly allow the straps to slide off the shoulders. Therefore you can best sew together both sides at the top of the zipper. Then opening the zipper will not give more slack to the straps, and you also have a hold to lock the zipper pull to. (see the first two illustrations) If you want use a lock on the zipper, it's pull should have a hole large enough to put the shackle of a padlock through. If not, you can use the generic techniques below. Depending on the agility of the wearer, and the tightness and stretchiness of the clothing, a back zipper can also be hard to undo by the wearer. A small padlock on the zipper of a jumpsuit with a back zipper, can often easily be hidden by hair falling down the back. That makes it suitable for public bondage. See also Ariel in Denim Skortalls and Ariel in Snug Overalls. Making 2 small holes in the fabric next to the zipper, you can put the padlock body on the inside of the garment, and have only the shackle, preferably through the zipper pull, visible on the outside. (see the third illustration) Also take a look at the contributor Jeansbound-DE, who shows how to use a metal collar through the zipper pull for another inconspicuous locking method.
YKK zipper pull with keys	Commercial zipper solutions There are several systems commercially available to lock zippers. The clips on the top 2 pictures are normally used with bags, but they can also be used on a jumpsuit. If the zipper runs up higher than this one, the clip might even be hidden by the collar. YKK sells a zipper pull with integrated locking mechanism. And even though the zipper pull is a bit larger than normal, it might not draw too much attention in everyday life, unless someone looks closely and sees the keyhole. An extra bonus of this pull is that you can still close the zipper with it locked, but not pull it down again. There are some more tips on using these zipper pulls below. See also Ariel in a Pink Sleeper.
	Laces Laces themselves are not so easy to use for locking: if you sew loops in them, or rings to them, they won't fit through the eyelets anymore. But with a little luck, those same eyelets come to your rescue: you can put a padlock through one or more strategic eyelets on both sides, and you're done.
	Belt You can use a belt at the waist as an additional fastening, but there are also jumpsuits with a wrapping top, that use a buckle to close. Put a padlock through a free hole of the belt beyond the buckle. You can include the buckle, as in the top illustration, but if the padlock is large enough not to fit through the buckle, that is not necessary, like in the bottom one. There also exist belts with a locking buckle, like those sometimes used in psychiatric hospitals. And locking leather bondage restraints often have a buckle where the pin ends in a loop, allowing a padlock to be put through there. But as long as you have a padlock of the right size, you don't need those fancy solutions, and with some skill, the padlock can be pretty inconspicuous, making it suitable for public bondage (esp. with a sweater over it). (see the middle illustration.) If you want to prevent the wearer to have access to certain vital parts, or visits to the bathroom, you can also lock the zipper of the fly to the belt, like in the bottom illustration. See also Ariel in Scenario: Retarded gallery 1.
	Generic Some fastenings, like zippers that don't have a hole in their pull, are hard to secure. But you can sew O- or D-rings on both sides, and lock those together. Pick one or two strategic places, like at the smallest of the waist and at the top. See for instance Ariel in a Red Maids Dress. If you put the rings and lock on the inside, they might even be hard to detect after the zipper is pulled up (depending on how tight the clothing is), making it more suitable for public bondage, though probably less comfortable. You can also try to use locking fasteners, like described at the end of the strap fastenings page.
	A complete restraint jumpsuit (548KB!) Here's an illustration I found on the Internet about a jumpsuit with a number of integrated restraints. The text is in German, but even without understanding the text, the illustration should be fairly clear. It uses a tab with Velcro and a Segufix lock to secure the zipper. The picture is made by Lord Grey (see also the Contributors section), but I don't think the design was ever actually made.

Locking Zipper Pulls

The fetish community is gradually discovering these zipper sliders with integrated lock: they are easy to use, and not drawing much attention. Of course the key mechanism is primitive, so it will probably be easy to pick, but then again, a pair of scissors will get you out of most things as well.
You can find them on boots, like those of Refuse To Be Usual (see also this video). You can also find them as an option on latex clothing like catsuits. And on onesies; even a non-fetish store like naturtraum-shop (German) sells them as an option, and some of their regular product photo's show them. (Another option is a back zipper.) They are apparently special needs-aware, but not aimed at fetishists. They don't seem to ask questions though if you order one with the options.

If you want to make your own clothing/boots/... lockable with such a pull, you can buy these pulls separately and replace the original pull. This is a bit fiddly but defionitely doable.
The original pull from YKK is not that easy to find in shops, but this shop sells them. There are Chinese clones too that you should be able to find on stores like eBay or Aliexpress.

Important is to realize that these pulls are meant only for 5mm plastic coil zippers. (Luckily though these are the most common sort.) You can see clear examples of the different types of zipper at YKK. I have heard that they might work on other types too, as long as they are 5mm wide, but I have no experience with that and no details on which other types could work. (Let me know if you have more information on that.)

Replacing the pull is pretty easy if you have a split zipper, like on a jacket; most often you can just slide the pull up along the one side and it will come free at the top. Then slide the locking pull the same way back, and ready.
The zipper halves on most one piece clothes are connected at the bottom. This makes replacing the pull harder, but there are enough tutorials on the web that help you on how to do that.