Some fundas i have learnt the hard way on the stock market:

a) Please Please dont invest in non-profit making companies.
b) I have found P/E to be a good indicator for investing. Other investing variables - NP, Dividend Payout, Previous Quarter Earnings, P/E of industry, Any recent news, Current Fancy of the stock market, Sales Margin.
c) Book profits. Book Profits. Book Profits at each opportunity. Have a rule ( say i book 25% profits when the share price increases 25% or whatever), and book profits. Cash profits are real ;-). I can quote my own case wherin we had a portfolio of 10 stocks (3 months back) for an investment of X Rs. It actually went up to almost 1.75 X, but we booked profits at 1.4X. I may be ruing the fact that i could have got 1.75X, but now the value is 0.75X. So the moral of the story - book profits at every possible interval.
( I will update this whenever i go wrong, right or whatever!)