Here you get a little thing, which is highly effective.
You start out, by buying a disposable plastic syringe, I guess that app. 3CC will be a good size for most purposes. Otherwise you'll have to find the one, that meet YOUR demands.
Having purchased the syringe, you carefully cut away the tip, meant to hold
the needle, and DO make sure, to remove any sharp edges and so on.
Having cut away the tip, you place the syringe on i.e. a nipple, and pull back
the piston rod, until she looks satisfied. Here you place a dot on the piston
rod (using a permanent fine line marker), where the piston rod comes out of the
syringe.
Having removed the syringe from the nipple, you drill a small hole in the piston
rod, app. 2 millimeters wide. The reason that the hole needs to be this small,
is simply, that the bigger the hole, the weaker the piston rod, and if it
breaks, you'd have spoiled the fun...
Finally, you grab i.e. a common paper-clip, and straighten it.
Now you place the syringe on the other nipple, pull back the piston rod, upon which you insert the paper-clip through the hole, thus preventing the piston, from sliding back. By now you have the syringe sucked steadily onto her nipple !
This little device, can also be used on her clit, but you'd most probably need another hole in the piston rod, as the clit IS more sensible, than a nipple !
Yet another little devilish thing, is to use the small rubber-rings, that you can buy in a shop selling dentist's supplies. You simply roll two rings up, over the syringe, suck out her nipple, upon which you push one of the rubber rings off the syringe. This will leave the rubber ring at the base of her nipple, which will increase the sensitivity, as well as it will make the nipple go larger and harder. (Though not permanently). Next you simply repeat the process, wit the other breast..!