It depends on how sensitive you are after an orgasm, really. I tend to enjoy the feeling of being overstimulated, and orgasm again quickly after a first. For a lot of people, however, that feeling isn't a turn-on. If that's the case, I'd say try one type of stimulation and then another, or try changing positions, switching from a toy to your fingers (or your partner's mouth!), etc.

You can't cause it intentionally, if it doesn't happen on its own. I could only have multiples regularly after I was 30 and had had a couple of kids. When I was younger, I would get sensitive and couldn't continue stimulation.

I don't have any suggestions, really. The first few times it happened, I had the first one or so from oral sex, then we'd switch to PIV and I'd have more. Now, we usually have oral sex and anal play, and then I need my vibrator to have the first one, then he goes down and I have more. It's a .... process.....

When I'm alone and want more than one, I usually have to have them in rapid succession, or I cool down.

Like P'Gell, I never had multiples until I hit my 30's. (Hell, I didn't even orgasm on a regular basis for quite some time!) I haven't had kids though.

What works for me is to make sure my clit doesn't get over-stimulated and overly sensitive. I still need stimulation there to orgasm, though, just not too much. Then for my second (and third, fourth, fifth, sixth, etc.) I like to focus mainly on internal g-spot stimulation until I'm inches away from orgasm, and then toss in just a bit of clitoral stimulation to push me over the wall.

I'm not sure that it's something everyone can do, but just because you can't do it now doesn't mean you won't be able to do it later - if that makes sense. Your body is constantly changing, and as long as you keep experimenting you'll continue to learn with it what it's capable of.