Gopi Krishna is the canonical worst-case scenario of an unprepared Kundalini awakening, so check out his "Kundalini: Evolutionary Energy in Man". I've heard that if you're careful, you should have an easier ride. However some people can have spontaneous awakenings, sometimes following traumatic (near death) events. One guy suggested there was a link between the adrenals and kicking off this process. The endocrine system is certainly involved.

But in general, symptoms can include some/all of the following, and vary in severity depending on how slowly you progress, how clean your lifestyle, etc:

* Energy surges - intense feelings of heat/electrical sensations in the body, typically in the spine (I've been woken in the past with electrical sensations surging through my body)
* Dietary issues - one of the nerve passes near the stomach and apparently can alter appetite as well as making you intolerant to some foods (I read of one guy not being able to keep water down for several days), etc.
* Sexual side effects - feeling intensely horny or totally disinterested
* Insomnia
* Headaches, feelings of congestion
* Hallucinations - auditory, visual, kinesthetic
* Spontaneous out of body experiences

Apparently these all settle down over time but can be a bit unsettling (e.g being woken in the night to ghoulish laughter, but there's nobody there).

On the plus side:

* Feelings of intense bliss/ecstasy, contentment
* Mental clarity

I've only experienced a few of the above negative side effects, and generally only mildly. But the Supreme Fire exercise has kicked things up a gear. Plus, I'm almost able to perform "full khechari" (look it up). My body just wants to do it. Spontaneous yoga can happen sometimes. Your body knows what it wants given half a chance :-)

As for joint damage - yes that is a definite risk. Never force the lotus position since it can put pressure laterally on your knees which can damage them over time. Also, there's debate around inversions (who'd have thought putting your entire body weight through your cervical vertebrae could be damaging :-D?), so while some people say they're amazing, I avoid them.

As it turns out, one of the most useful sitting postures places direct pressure on the perineum. This is traditionally done with the heel (siddhasana) but can be done with a prosthetic. Roll a sock up and sit on that. I use that on a Zen-style meditation bench to spare my knees sometimes. So you don't need great flexibility, certainly not to the level of the hyper-flexible people in yoga magazines.

A lot of it is common sense - don't force yourself with either the physical postures, or breath retention, and listen to your body. You need to activate the parasympathetic nervous system and operate in that mode. The other thing to bear in mind is you can have a delayed reaction. So you might do some breath retention, and feel great. So you do more. And more. Then that night you get the congestion/headaches/insomnia. So just build up gradually over several weeks, and back off if you need to. It's no different to pacing yourself with weights at the gym.

The AYP site has a very helpful forum with people far further down the road than I am. If you're interested you'll find lots of information on that forum to peruse at your leisure, and a friendly community to answer questions.