I recently did the following:
I chose specific people with whom I've had recent contact and who, collectively, provide some breadth. (E.g. I'd rather have recommendations from an interaction designer, a senior developer, and a product manager than just three developers.) I also sent a request to someone I'd previously provided a (solicited) recommendation for.
I used the LinkedIn interface to send the requests but I wrote personal messages to each.
In those messages I said what areas I hoped that particular person would be able to address (while saying that of course I'd welcome a recommendation on anything they cared to write about). I did this partly because of the considerations in #1 and partly to overcome the "um, what should I write?" problem.
I did not send them all at once, because I don't want to give linked coworkers (who get those weekly updates) the impression that I'm actively looking.
For people you haven't stayed in touch with, put yourself in the other person's shoes and ask yourself: how will this person respond to this request out of the blue? If you had a very strong working relationship years ago but haven't stayed in touch, the person might remember you fondly and be willing to write one (but see below). If he's likely to be asking himself "who is that again?", then he's probably going to delete the email.
But even if the relationship was strong and you think he'll write a recommendation, it's better to actually get in touch first yourself, outside of LinkedIn. Send email, ask about his {family, pet, hobby he always talked about, etc}, share something he might be interested in if you can (such as a pointer to an article on something he's passionate about), and only then request the reommendation.
If you didn't have a strong relationship to begin with, then asking for the recommendation is premature, and you'll have to start with getting back in touch, see where the conversation leads, and decide later if asking for a recommendation is appropriate.