G-spot stimulation typically achieves it for most ladies. Some pair it with clitoral stimulation (I need both 90% of the time). I personally prefer a standard dildo with a curve over the ones that have a very narrow shaft with a G-spot bulb at the end, but others may prefer the bulb types. It's a little different for everyone, since every person's anatomy is slightly different, so try a few different techniques and see what you can make happen.
Also, waterproof blankets. Liberator makes several. They're not very absorbent, so you may still want to put a towel on top, but the surface under you will stay bone dry.