The best protection is to be grounded, actually. By being grounded the clients ills will tend to stay with them and be transformed by the client. By being grounded you will be more full of energy, and your own essence, which will naturally cause the energy flow to be from you into the client, rather than from the client into you.

As a healer one of the common things that are done is to take the clients ills within oneself. This happened vividly to me once; my client was experiencing some deep old and vivid trauma, my guidance was saying to reach inside her and touch that trauma, I did and all of a sudden there was a swoosh and my client suddenly stopped experiencing the trauma. Instead her trauma went into me. It's just energy, after all, and can go from one place to another very easily.

Many ancient traditional forms of energy healing use this practice. If you remember the Star Trek episode "The Empath", this was a great example of this style of healing. If your past lives include training in this style of healing, then you might employ it yourself in todays healings.

In those traditional empathic forms of healing, the healer is supposed to take that energy and transform it themselves. If you do not do this, because maybe you haven't remembered from those past lives this is the second half to that style of healing, then maybe you will take on the illnesses your clients have. By all means it is not good form for the healer to get sick, and maybe die, from giving healing to their clients. The world needs healers who are well, not healers who are dying.

With Reiki it is quite possible to treat oneself as well as others.  Treating oneself is simply placing your own hands on yourself and intending for the energy to run.  By treating onself you can more quickly know what the experience of receiving Reiki is like.

Comfort is sometimes harder to arrange depending on where the energy is needed.  Being inventive most of the places in ones body can be reached.  For those body parts which cannot be reached one can try the remote healing techniques to do a remote healing to yourself.