Your body is the first and last thing you possess in this lifetime. It is the machinery through which you enact your will upon the physical realm. Thus, few things carry your essence as strongly as pieces of you. For this reason, among others, the use of bodily fluids in spell casting - particularly blood - is hotly debated.
Many reject its use as a way of distancing themselves from ancient and 'barbaric' forms of magic. Others perceive the body as a sacred vessel, and therefore consider the use of its parts a form of 'heresy,' for lack of a better word. Others still just find the concept intuitively revolting.
Despite these and other objections, some spell casters - myself included - still use blood in minute amounts to directly insert their essence into their magical goings-on. If you are currently or would like to use blood in a ritual, it is important to treat it with the respect it deserves ... not only as an extension of your bodily temple, but also as a potential agent for infectious disease.
Here are some guidelines for safely drawing blood.
1. The handling of bodily fluids always poses the risk of contracting disease. For your safety and the safety of others, do not allow others to handle your blood, and do not handle others' blood. If you must handle someone else's blood, wear gloves and thoroughly wash your hands when you're done.
2. Those who suffer from hemophilia, hemophobia, Von Willebrand disease, and any other disorder - physical or mental - associated with blood or blood clotting should never endeavor to harvest it.
3. Always keep a first aid kit handy. It should contain an antiseptic/antibacterial agent to prevent infection, and gauze or bandages to protect the wound as it heals.
4. Always make sure your cutting implement is sterilized properly before use. To sterilize a knife, you may either place it in boiling water for five minutes, or submerge it in rubbing alcohol or hydrogen peroxide. Always wash your hands before handling the knife again so as not to re-contaminate it.
5. To numb the area where the incision is to be made, hold an icecube to it. This will reduce (if not completely eliminate) any pain.
6. To draw blood, make an incision only as deep and as large as an average paper cut. Slice diagonally to just barely break the skin. Then, gently squeeze the sides of your fingertip to produce droplets.
Spells that use blood harness only minute amounts, so any further cutting is unnecessary and highly dangerous. Never draw blood from veins or arteries, as this can lead to exsanguination (bleeding out). If you are bleeding copiously, call an emergency medical service IMMEDIATELY.
7. Blood coagulates and dries. It will not retain its fluid quality once it has left your body.
8. To ensure that objects stained with blood are properly and safely disposed of:
a. If the object is made of cloth, burn it in a fireproof container. You'll want to do this someplace private so as not to draw attention to yourself. If you're burning it indoors, do so in a well-ventilated room far from fire detectors.
b. If the object is not flammable, seal it in seran wrap and place it in a tightly-sealed container before disposal to prevent any subsequent cross-contanimation. If you can, dispose of it in a waste receptacle that is clearly marked as containing hazardous materials.
The preparation involved may go above and beyond the usual for casting spells, but it is worth it to ensure that the practice is safe, sterile, harmless and discreet. If it cannot be done right, it ought not be done at all ... and when it's done right, it can greatly improve the outcome of certain spells and rituals by (literally) giving a little of yourself to the process.