Anamchara has a good post referencing an introduction by Emilie Griffin to Evelyn Underhill: Essential Writings (Orbis). She points out five characteristics of genuine Christian mysticism:

• Christian mysticism is active and practical...for the vast majority of Christian contemplatives, the life of silence is embedded in a network of community relationships...True mysticism does not fly from such obligations, but embraces them and seeks to meet them well.
• Christian mysticism is spiritual and transcendental, rather than magical. The authentic mystic does not seek supernatural power for the purpose of controlling earthly circumstances, but rather seeks to surrender to the will and calling of Divine Love...
• Christian mysticism is centered in love
• Union with God in authentic mysticism transforms the mystic for ever richer levels of life...Mysticism points beyond itself to the life of kenosis and theosis: self-emptying in order to participate in the Divine nature.
• As a result of such loving union, the authentic mystic becomes unselfish.