You abuse the trout's confidence. Really.

I learned to lay on a bank above a spot where you could see the trout (keeping your shadow out of the water and approaching stealthfully, of course). SLOOOOOOOOOOOOOOOWLY lower you hand and arm into the water behind the trout sloooooooooowly and carefully work your way up until your under the trout's belly -- which you sloooowly and carefully stoke front to back (like petting a cat). This relaxes the trout and puts it off guard, and then on one of the strokes you catch it and flip it up onto the bank. Wait a bit and do it again. Trout aren't very smart ("Gosh, where'd Doug go?" "Dunno, but his feeding spot is open and I got dibs!")