I try to pick stocks I won't sell for at least a year...preferably that I won't sell for at least five years or will virtually never sell. I don't have the time to research continually, and frankly I think the only way an amateur can do well at this is to put the research in at the beginning and then buy a stock that will (hopefully) experience gradual appreciation both in good markets and bad. Ie., I don't waste time with yearly price targets and gimmicky 'plays.' I also reinvest dividends on all of my stocks. Between taxes, commissions, and my evaluation of the ratio of my risk/reward payoff to the amount of work I'd need to do, this seems like the only sensible way for a small-fry like me to invest (other than using mutual funds and my 401k of course).
HOWEVER, I have also found that certain stocks overperform so much in a short time, that one must sell them (even if one loves the company), because they have nowhere to go but down, in which case one may ride a low-growth cycle in that stock when one could be making money elsewhere. Examples would be CCF, which I sold at $31/share, TM, which I sold at $130/ADR. I'm starting to feel that way now about BHP....