Moving S L O W L Y is key. Even turning your head needs to be deliberate and slow. When you are overtaken by a snail, you are almost moving slow enough not to be noticed by a deer. It takes time to look where your foot will fall, and then look up to scan for deer.... and repeat.... and repeat....
Edit: of course, you can't spook deer that aren't there, so making sure that deer are present is quite important too.
And don't underestimate luck... As they say, the best days' hunting are always yesterday and tomorrow...
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