First off, get the "bestiality" off the board, this is a family place.....what the heck is that bear doing, anyway...
Since we like to be pretty sure before entering a trade, we get in with our normal share size (say 2,000 in my case). If I were to only want to buy 1,000, I look at it like "maybe I'm wrong" - so we tend to enter with the whole chunk. Hey, if we're wrong, we can get out just as quickly, and even flip to a reverse postion.
I am guilty of "scaling out" of positions...taking some profits....with this in mind. If I buy 2000 shares, it goes up 50 cents, I sell 1000, and if it goes back a quarter or so, but the market looks good, I may just buy the ripple....and be back in for the 2000. If it were to continue up after selling half, I'm still happy (although I have "lost" half of the additional profits)...kind of a "glass half full or empty" scenerio.
Most importantly -- you need to be in the trade with a quantity that makes you very serious, without being "freaked out" - don't ever do too big of size....(didn't expect to hear that from a "firm" guy, did you)...but it's true, traders tend to freeze when they have too big of a position.