I have some quality serrated knives but over time they get dull.
How do I sharpen them?
Take them to a professional. Nothing you can affordably buy in your home will work well.
Personally, I don't buy quality serrated knives. I buy cheap and replace when dull. Only my normal blades are quality, and these I have sharpened yearly.
dmckee is right - that you can use a rod & file to fix - but that's a HUGE pain and very difficult to do. not preferred unless absolutely necessary.
michael has a point, that some electric sharpeners allow you to hone serrated edges. this is because those machines use a flexible rubber wheel on the honing stage. this doesn't correct misaligned teeth though. it will help.
you CAN use a stone to re-align the flat side, if your teeth are bent on that side, that can also help - but again, requires great care and skill.
the best solution is to NOT damage the teeth to begin with. don't cut on glass / stone. use a knife block or store the serrated blade in a sheath. don't EVER dump the blade in with other knives, the teeth are easily damaged.
if you take care of it, a serrated blade will last you a lifetime.
The Chefs Choice 130, which I absolutely love, can hone serrated knives on the third (polishing) stone only. This is enough to improve the cutting significantly, without having to take it out for a professional sharpening.
In my experience, serrated knives are sculpted from one side of the bevel only. The other is flat. I just hone mine on a fine oil-stone, using a stream of water at the sink faucet for lubrication. My stone's mounted on a wooden paddle so it's easy to use for sharpening kitchen knives. Yes, I'm probably just sharpening the tips of the serrated edge, not the gullets, but that's the part of the knife that does most of the work and needs it most. You just need a bit of practice with a honing stone to be quick and effective in restoring your knife edges to keenness.
I have experimented with a rat-tail file for course work and a straighting rod for fine (basically following my old boy-scout instructions for knives and axes). Very labor intensive as you have to do each serration separately.
The results were better than nothing, but not particularly good.
It you are going to try it, you will need to find a file with a diameter that matches the serration.
I hone it with a steel. I was a little surprised the first time I saw someone do this but it really works.