How can I tighten drawer knobs?

The machine screws might be too short to get a good grip on the knobs. If so, you'll need to buy longer screws. Be sure to take a knob to the store because the threads have to match exactly. If you can't find screws of the right length, buy long bolts and cut them with a hacksaw. Always put a nut on the screw before you cut; it will realign any damaged threads when you twist it off. Or save yourself that tedium and use truss-head breakaway screws. The heads are large and flat, so they won't pull through the wood and loosen, as smaller-head screws sometimes do. Their shanks are scored in ¼-inch increments so that you can snap off what you don't need without damaging the threads. I've found that the bolts won't always snap where you want them to, so I snip them at the score using lineman's pliers.

If the knobs still loosen, put a drop of thread-locking liquid on the screw threads before you install the knobs. Mechanics use this to prevent bolts from vibrating loose. It's like a weak glue, so it holds the parts together but not so tightly that they can't be loosened later if you need to remove them.

Use utility shears to cut the toothpicks flush with the back of the drawer knob. Sand the ends of the toothpicks, if necessary, to ensure they sit flush with the back of the knob. Hold the screw in place with one hand while you hand-tighten the knob onto the screw.

You could use a bit of glue to secure them.

Get a tube of Locktite at the hardware store or autoparts store. It only takes a drop to do the trick.

A rubber washer might also work. Ceramic knobs can break if you tighten them too much.