Question: My toddler doesn't like to get his teeth brushed. How do I get him to be more cooperative?

He knows it's part of the bedtime routine and doesn't try to avoid it. He will happily grab and hold out his toothbrush for us to put toothpaste on it, and then he enjoys sucking and chewing it. 

When he's done that for a short while, we decide it's necessary to do it properly, and that often ends in me holding him while my wife brushing his teeth, while we explain what we're doing and why. It only takes 20 seconds, but it's always accompanied by screaming and squirming. I'd like to find a way to avoid this. As soon as we're done, everything is fine again.

**Clarification and update:**  

At this age, we don't expect him to be able to brush his teeth on his own. What I meant was that he didn't allow us to do it for him either. 

In the meantime, four months have passed since I asked this, and he's developed a lot. Now he gladly lets us brush his teeth carefully and cheerfully. 

**Update 2:**  

Another 6 months have passed, and he's still not enjoying the experience. He understands that it has to be done, and he sees us do it too. Sometimes he'll happily let us brush his teeth, other times it's a struggle. 

We are using several suggestions from the answers below -- we sing, we distract, we talk about "dirty teeth" and brushing the "dirty sugar" away, we let him brush on his own for a while and then take over, he can see himself in the mirror, etc.
All of this helps, but one factor certainly is that he's tired, and that's natural because brushing teeth is part of the bedtime routine.

Answer: 
Both of ours went through a stage like this (in fact number two is still going through it). 

* Lighten up - if you don't get a perfect brush in EVERY day, nothing bad is going to happen
* Make sure it really is about the teeth and not power struggles
* Try a mixed approach - if one idea works for you, great, but have a whole load of others ready!

Making a game out of their resistance worked (sometimes) for me: I'd poke the brush at the, and they'd turn their mouth away, but every now and then (because they're laughing) I'd 'hit'. Generally, after a few minutes of that and them laughing silly, they'd relax and let me 'win' .. a bit. Sometimes.
I was watching The Two Towers one day, while my daughter was watching a princess movie in the other room. She wandered in and started watching The Two Towers with me. There were some orcs on the screen, and it struck me. "Addison, see those guys? They're called orcs. See their teeth? They're yucky aren't they? That's what happens when you don't brush your teeth."

We haven't had too much of a problem since then, and she occasionally attempts to explain to her little brother that his teeth will look like an orc's if he doesn't let us brush them.

(Luckily, she didn't have nightmares about said orcs either)