In fact, the coffee enemas are used to clean the blood serum. Portal veins run from the large intestine to the liver, when they absorb the coffee the liver is stimulated and starts to clean the blood with much greater efficiency. The resultant toxic waste is flushed into the small intestine via the bile gland.
If you can take a coffee directly, then there is no need for a cleansing enema. After evacuating the coffee, there should neither be a need for a post cleansing enema. Any of the bile waste fluid that is not expelled can be easily dealt with by the kidneys.
It is more important to make sure a few toxins as possible enter the body, and to drink plenty of organic vegetable/fruit juice to give the body something to rebuild by.
And to only use distilled water for the enema coffee fluid (and light-roasted organic coffee beans). coffee enemas are used by many, with no reports of them actually causing constipation - and they will relieve (eliminate) constipation (at least they do for me).
As far as additives go, personally I will not put anything up one end, that i would not also put in the other. It just seems common sense - although the large intestine is much better at toxin removal than the mouth/stomach/small intestine, why give it toxins in the first place ?(including salt - not a toxin, rather a toxin promoter).
Someone mentioned food grade hydrogen peroxide - I have had great success using that, but be very careful about the dosing/length of time using. Probiotics also seem useful.