I love the sound of a guitar played through wah wah as much as the next guy, but I have some problems picking that skill up. I would love if anyone can either give me some tips on how to start using the pedal properly or refer me to a book, dvd, article that shows exercises, especially on funky licks, including strumming muted stirngs.
People use Wah in lots of ways; for me I find that the most effective way to use it is to wah with the feel of what your doing. I have seen many people just rock back and forwards on the things at the same speed (usually fast) no matter what they are playing, that what you don't want to do. Remember its an expression pedal.
Here's a few examples:
If your bending a note up a step; try rocking forward on the wah at the same speed as your bending the note.
If you change the speed of a bend or double bend try matching it with the wah,
Try strumming the with the strings slightly dampened with your fretting hand, rocking on the wah to the rhythm.
Pretty much anything really but stay expressive; try trilling a note or two while doing different things with the wah.
Or just find a sweet spot on the wah and leave it on there (no rocking) while jamming.
You'll soon get the hang of it.
To counter the Tin Man (who gave a fine opinionated answer, but one that I feel needs to be balanced!) a wah works very well as a rhythm device, and I personally love the sound of it as heard on many funk and disco classics. In fact the most recent song I've finished used a wah in exactly this way.
Although I admit it helps to not just alternate it open/closed metronomically. But if you work out a rhythm pattern and repeat it, and combine that with some delay, it can be hypnotically groovy. To make dance music, you have to be effectively "dancing" as you play, so your foot becomes part of that motion.
A wah is not a rhythm device, but instead is a tool to bring out a vocal quality in the guitar's sound. This is in spite of what was played on a lot of motown and disco hits. :-)
In his Derek and the Dominos days, Clapton used his wah a lot. Listen to what he plays on the "Live at the Fillmore" album, for how one should be played. No one song is a better choice than another because each time he stepped on it it was talking.
Jeff Beck is another one who I think uses it the way it should be used. The intro to "Hammerhead" from the "Live and Exclusive at the Grammy Museum" album talks away.