How do you join programme committees for conferences?

It's rare. I know of one conference though (in theoretical computer science) where the steering committee actively maintains a list of "qualified but has never served", and strongly encourages PC chairs to look over that list when selecting names. Your best strategy might be to approach the steering committee and suggest that they do something like this, so as not to sound like you're shilling for yourself :)

I agree with Suresh, it's rare, I've never seen a call for invitation to a PC in my field (Computer Science). I have actually seen people making fun of emails asking to be in a PC (i.e. if you ask the PC Chair to join the PC, it could be negatively perceived).

From what I understand, there are at least five possibilities to get a PC chair to invite you to join a PC (of course, none of them are automatic, they can just be helpful. In the end, the PC Chair makes his own choices, in order to have a PC that is attractive enough for authors to submit a paper, and serious enough so that the reviewing process can be done effectively):

• if you were in the PC of the previous editions
• if you know the PC Chair, so that he can trust you to do your job,
• if you already are in other PCs of conferences in a related area (i.e. other PC Chairs trusted you)
• if you have published several papers in previous editions of a conference
• if someone you know is invited to join the PC, but he can't and suggests you.

Of course, it's a cycle: the higher reputation you have, the higher are your chances to be invited to join a PC, and the more PC you join, the higher is your reputation.

Talk to members of the current organizing committee? It's tough work, and scientists are always busy, so they'll usually be receptive to having someone else to split the work with, and there's always a chance that somebody is too busy to help with next year's committee, and they're looking for a replacement.