if you're finding yourself upside down in stocks more often than you'd like, i recommend making a plan for every trade and sticking to it, no matter what. when you buy in, tell yourself you'll sell if it gets to X. put a stop in, and move it higher as the stock rises. if it drops you can stay where you are, or move your stop up to conserve capital.
imo it's helpful to practice taking small losses so you get a stomach for it. this is self-correcting, because pretty soon you'll time your entries better.
the next step is to learn how to sell too soon. seriously! practice taking a modest profit, instead of swinging for the fences, and then moving on to the next one. do this several times a day and pretty soon it really starts to add up. :)