Ive done it with Psychotria 'Nexus' before. Just left in a cup of water inside a bag for humidity. The cutting actually started flowering whilst it was in there and rooting at the same time!
Ive currently got some Psychotria 'DW06' stem cuttings in water, ive not got these in a bag this time as I ran out, however they seem to be doing fine like this and its not seeming necessary for them to be in one.
Ive tried stem cuttings of Psychotria kirkii, probably 3 or 4 with each of these methods - water rooting like above, rooting in soil and rooting in Sphagnum moss. I only had 1 cutting which rooted and that was in soil. Very tricky to root in my experience.