INVESTIGATION GUIDELINES from the SPR:
1. Do not go by yourself.
2. Keep your relations with the experients/witnesses as brief and relaxed as possible.
3. keep an open mind.
4. be tactful, or even reticent, in expressing your views.
5. Do not play the amateur psychiatrist.
6. Respect the confidentiality of the case.
7. Take particular care where children are involved.
8. Avoid publicity.
9. Learn from one's mistakes.
This seems to me like an excellent list of guidelines, given more impact (for me) by the knowledge that I have sometimes gone against some of this advice.
I also recently purchased and read Dr Ruickbie's book (Book Depository link above) and also his 'A brief guide to the supernatural'. Both books are excellent reading, highly informative, and will make excellent reference sources if I ever manage to travel to the UK again.