This is something that I am still working on. As with everything, it helps if the person is open to the idea of distant healing. Though sometimes we feel compelled to send healing even when we are unsure whether such an action would be appreciated, that is when you can ask that what we send out is for the greater good.
From what I have experienced myself, and to talking to those that do this regularly, it comes down to INTENT. Whether it be just a little boost of energy to help a friend get through the day, or a full on crystal healing in which I visualise that I am placing the crystals on their chakra's as if the person were laying right in front of me and concentrating energy for healing where I feel the body needs it.
I have also heard anecdotal evidence of Reiki healers who have been asked to send energy and healing, and have thought "Yes I will do that", but than have got side tracked or forget to set time aside to do it, only to hear back from the person that made the request that they felt the healing energy
and doing much better. It is with this anecdotal evidence that they have realised that their INTENT to do so, was enough for the healing energy to be sent to the person that required the healing.
Whilst I am still new to distant healing and crystals, I do hope to do a Reiki course when the time is right, until than I am practising the Middle Pillar energy flow, to better connect with the universal energy of the divine.