That's really easy for most traders who know their stuff. If your trading a small part of your equity there is no emotion so you really can't lose a trade unless you try. It's the emotion that gets people.
All in All out is the proper method. However all in does not mean your entire liquid net worth. It simply means all in for the amount that you would like to commit to the trade/idea. Do not average in or average out (scale in scale out) ever. Get right.