In NU usage, it's important to make sure that Swalot doesn't become a sub-par Muk. If you want to use Swalot, you need to use it in a way that Muk or other Pokemon can't function. As such, I believe there's only one proper set to use for it (and this set is probably Swalot's best set for use in OU).
Ability: Sticky Hold
Item: Leftovers/Black Sludge
Nature: Careful (+SpD,-SAtk)/Calm (+SpD,-Atk)
EVs: 4 HP/252 Def/252 SDef
Earthquake/Fire Punch/Ice Beam/Shadow Ball/HP Fire
This set obviously takes a lot of predictive skill, but using Swalot is already hard as it is, so you have to take a bit of risk to be successful. You should use Encore on a Pokemon setting up entry hazards, raising stats, etc. You can pick a status depending on what you're trying to do. You can use Toxic to wear down other walls, Body Slam can heavily cripple sweepers trying to take Swalot down, and Yawn can force switches, leading to entry hazard damage and maybe a chance to use Pain Split on a fresh Pokemon if you need more health. Pain Split is a pretty unreliable form of recovery, but when compared to Rest as its only alternative, at least it's something.
The final moveslot should go to a damage-inducing attack. This really depends on what you're worried about. If you're running Toxic, Earthquake and Fire Punch (with a Careful nature) are advised in order to deal with Steel-types that would otherwise brush Toxic off. HP Fire can be used for this if you're running a Calm nature. However, if you're running a calm nature, you're better off using Ice Beam or Shadow Ball. Ice Beam lets you hit Gliscor (OU), Altaria and Scyther (UU), and basically any Ground-types really hard that would attempt to threaten Swalot. Shadow Ball can also hit Gengar and all of Rotom's forms hard. If you're running Ice Beam or Shadow Ball, be sure to watch out for Steel-types, as they'll wall you all day.
If you want to run two damage-inducing attacks (generally Earthquake and Fire Punch, or two of Ice Beam, Shadow Ball, and HP Fire), you could possibly run Yawn in the place of Encore to screw with an opponent that might be second-guessing using a stat-boosting move with Swalot in its presence. You can also forgo Pain Split in order to run a second damage-inducing move, though it isn't advisable, seeing as Swalot takes on a lot of damage over time.
The two most important things to remember about Swalot is that it needs to avoid being a subpar Muk, and it needs to be able to deal with Steel-types and Ground-types, especially the former if you're running Toxic. If you keep an eye out for these guys, Swalot can be a reasonable wall.
Hope that wasn't too long winded.