These are nylon doohickies that fit onto the top of the rod and press against the rim. They usually work, but have been known to strip out over time. Another idea is to apply a small drop of ThreadLock to the rods' threads after they've been tuned. This will prevent the rods from vibrating loose, but easily breaks free if you need to tune. You can probably figure out why.
Many drum manufacturers use springs inside the lug casings to hold the receiver in place.
These small, seemingly isolated springs can wreak havoc on a drum's sound, especially in the studio. They cause annoying buzzes and rings. If your lugs contain springs, you have a couple options. Replace them with flexible plastic or rubber tubing. Most any hardware store has an ample supply of this.