First of all, I trust the Gods. Even though I know there are many spirits who have their own questionable agendas I put faith in the Gods, and by extension the Lwa, to be in keeping with Universal Will. If I am aligning myself to the Gods I am also attuning myself to that Universal Will, which is by extension my own True Will.
"But, Glaux," you ask, "How can a trickster deity like Loki, or Discordia, or Papa Ghede be keeping with Universal Will? Isn't that like inviting in Charonzon?" Yes and no. Even Charonzon has a place and a purpose in the multiverse. By aligning myself with the energy and Will of Chaos, Death, or the Adversary I am bringing their balancing energy into our realm.
Also, and this is an even bigger reason than aligning myself with Universal Will, I 'm a witch. A Thelemic witch, yes, but firstly a witch. That is my Great Work. Those are my blood oaths. I walk a crooked path. I talk with Gods, and invite them in. I bring about change within myself and the world around me by working with spirits of the Gods, the Ancestors, the Mighty Dead, and the Seelie & Unseelie Courts. Possession is a key technique for a shaman, and that is what witchcraft is, the vestiges of ancient European shamanism.
Maybe you think that witchcraft and Thelema are at odds with each other because of how each system views possession, but I maintain that witchcraft, specifically Gardnerian Wicca is the most prolific expression of Thelema in the New Aeon. It is no secret that Gardner had a charter from Crowley, or that the original Gardnerian Book of Shadows borrowed (stole?) heavily from the Gnostic Mass and other Thelemic texts. Nor should it come as a surprise that Wicca has pollinated and disseminated itself through mainstream culture much more effectively than the magical system of Thelema. Wicca is the most popular face of Thelema today, even if most witches have never heard of Crowley or the Book of the Law. So, no, I don't think that witchcraft and Thelema are mutually exclusive paths, rather I believe that, for those who are bound to each system, they compliment each other as magical paths.