Pick one faith and stay with it. Dilettantism is the mark of an amateur.
Familiarize yourself with the specifications for sacrificial victims, and ensure that unacceptable substitutes cannot be unexpectedly introduced into the ceremony. If the penalty for not-to-specs work is death and/or mutilation, consider working for a more fault-tolerant deity.
Avoid needless embarrassment. Practice the correct pronunciation of your deity's name in the privacy of your own room before chanting it in public. Flash cards are often helpful.
Before agreeing to impregnation by a supernatural being, investigate the survival rate of the other women who have undergone the procedure.
Never invoke anything bigger than your head.
Eschew deities whose followers are all young; such faith groups usually employ an unpleasant retirement procedure.
Avoid all cabalistic jewelry over ten pounds in weight -- it attracts unwelcome attention from tourists, policemen, various supernatural creatures, and can be downright dangerous during thunderstorms. Its jingling also tends to warn the hero of your approach.
Citronella candles may not be used in rituals. I cannot stress this enough. Pastel colored candles in the shape of cute animals are like direct sunlight to the Powers of Darkness.
If the spirit contacted during a seance begins offering financial advice, you're dealing with a con artist, and not a genuine medium.
Always keep your kit with you: candles, chalk, incense, silver knife, Thuggee cord, service revolver, garlic, and cab fare.
Fluorescent lighting is very annoying to most netherworldly creatures.
When the Black Mass goes awry, stay away from the Evil Priest. Enraged demons always go for the pompous.
When a religious artifact begins emitting light, CLOSE YOUR EYES. Thousands of cult members could be saved every year if they followed this simple safety tip.
When mutilating cattle, avoid the ones with testicles.
During ritual sacrificing, taking bits home for later is now generally considered "bad form."
Contrary to historical belief, drugs and invocations do not mix. When the ritual goes awry, it is vitally necessary to be able to discern between the gibbering monstrosity to pump full of silver bullets and the gibbering monstrosity that will fade away after a few hours, some B complex, and a good hot bath.
Piety and belief are powerful things, and few forces in nature, can stand against one who is true to his faith, his god/goddess, and the deal made in exchange for the soul. However, it is also true that gods tend to side with the heaviest artillery, so be prepared to change sides at the drop of a hat.
For those situations where a fresh, living, sacrifice is not available, the lower ranks of demons can be fooled by microwaving a previously frozen chunk of ex-victim and cleverly jiggling it. However, a mock victim sculpted from SPAM is unacceptable.
Instead of picking human victims who are young, virginal and innocent (and tend to turn out to be the Hero's girl/boyfriend), see if you can substitute mass murderers, lawyers, or other people who won't be missed.