I love the sounds/unusual directions it's possible to take music in by employing polyrhythms:
http://en.wikipedia.org/wiki/Polyrhythm

Seems like you can make a fairly simple piece sound very busy and complex.....provided you can pull off polyrhythms.

Inbetween trying to promote an EP, preparing for gigs and building a mixing desk, I'm trying to learn how to do this well, on acoustic and/or piano.
I can pull it off on piano, slowly, but as soon as I deviate from simple things and actually try to put a melody of interest in, my counting screws up. Shifting from polyrhythms back to regular is also tricky.

Anyone got tips for practising this?

Not really haha  Listen to The Rite Of Spring by Stravinsky. There's plenty in there

I think Polyrhythm is the same as Counterpoint isn't it? I might be wrong. Counterpoint is something

I have been harping on about for ages 

I don't usually try to write Polyrhythmic Counterpoint in one part though. I usually have two or mare parts... Whether they be two or three guitar

parts or guitar and keyboards, to get that counterpoint going in some parts of my songs. Not all of them.

It is a very effective tool I agree 

Counterpoint isn't the same as polyrhythm, but counterpoint can be polyrhythmic. Counterpoint is a second melody that runs parallel to the main melody, while having an independent melodic voice of its own. Listen to Bach, and you're probably listening to counterpoint.

As for polyrhythm, the approach I tend to take is that if it happens naturally, I'll spend the time learning how to play it. But I don't consciously use these devices.

Yes, generally I let things happen naturally. But, polyrhythm never happens naturally as I cant do it well :p. If I can learn to control my hands better independently....
If I had polyrhythm down to a tee Im sure it would present itself naturally when songwriting...

It's like with your first songs...my first songs on guitar consisted of E A and D as thats the only chords I could play...but you learn the other stuff and it happens naturally