Fix a dented speaker cone
Last weekend I had a yard sale with a few pairs of speakers to unload. After the first wave of hard-core hagglers trampled through my yard I noticed that someone had trouble resisting the impulse to dent the center of my vintage speakers.
Since I was not about to let them spoil my day (or my profits), I rushed the speakers inside the house to fix them up. A quick online search for some DIY solutions bore some useful results, which I've gathered here.
Swabs and glue
OK, honestly, this technique bubbled up from the dark recesses of my memory and not the Internet, though I'm sure it's out there somewhere. It's also the technique that worked for me right off the bat.
You'll need a cotton swab, some scissors or nail clippers, and a couple dabs of Super Glue. Use the scissors to give one end of the cotton swap a flat top. This will give it a bigger surface area to make contact with the speaker. Next, put a few drops of the Super Glue on the flattened end of the swab you just cut, give it a second to soak in, and then press it firmly into the center of the dent. Hold it there for 15-30 seconds and when you release it, it should be able to stand up on its own.
One of the most popular techniques recommend on the Internet for removing a speaker dent is to vacuum it out with a hose attachment. I didn't attempt this one until later, as I was certain that the vacuum would rip the dome out of my speaker altogether.
But in my case, at least, the vacuum was a great solution. It's also the only technique on this list that won't leave behind glue residue or a puncture. You simply turn on your vacuum, place the hose attachment over the dent, and with any luck your dent will pop right back into shape.
Another variation on this technique is to use a an empty paper towel roll like a giant straw and suck the dent out like a milkshake, but pride prevented me from giving that one a shot.
Poke and pull
Finally, as an option of last resort, you can try piercing the center of the dent with a slightly bent sewing needle and carefully hook the needle around to pull out the dent. If you have a really nasty, complicated dent to undo, this can be a fine way to go -- but once you have a hole in your dome there's really no way to prettily patch it up. Your speaker will forever look like an eyeball with a little strung-out pupil in the middle. Sonically, though, a little pinhole in your dust cap isn't going to affect your sound quality. It's just ugly.
So there you go; those are my personally tested techniques for taking the dent out of a speaker dust cap. I also have a video that demonstrates each option, located at the top of this post.