The main problem with clitoral trodes is keeping them on. Medical gel pads work well, especially the first time they're used, since they stick really well then. I usally use these, with the other contact either on the perineum below the vaginal opening, or across the bottom of the labia majora. Another problem with all other methods is getting a contact area large enough to avoid a sharp unpleasant feeling, while still putting enough current through to have an intense effect.
Women often make their own clit trode from paper clips, long hair pins, curtain/drapery hooks -- often covering the metal with a length of thin Conductive Rubber tubing, which gives it a larger contact area & softens the feeling.
A vacuum cylinder with a contact can be good, but the main problem is getting a large enough contact area that will allow higher current without sharp burning feeling.