Downright Jokes

Avoid all esoteric jewellery over ten pounds in weight - it attracts unwelcome attention from muggers, policemen, various supernatural creatures and can be and are downright dangerous during thunderstorms.
Avoid using coloured candles in rituals. I cannot stress this enough. Pastel-coloured candles in the shape of cute animals are like beacons to the weirdest demons.
Never make flippant remarks to a demon ("Hey, Belial, you look like hell, ha ha."). It may retort with its own brand of humour, like tearing your limbs apart.
Always keep your kit with you: candles, chalk, incense, silver knife, service revolver, garlic, taxi fare, condoms, and change.
When the Black Mass goes awry, stay away from the High Priest. Enraged demons always go for the pompous.
If a demon promises you untold riches in exchange for your body, ask for an advance - freeloading sex fiends abound.
If the entity you summoned offers you its soul in return for money, chances are that more...