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There's a rumor going around that you won't find shaking grass until you beat the first gym leader. I didn't believe too much in it when I first got the game so I didn't bother with the Gym. Spent about a half hour looking and didn't find anything. Gave up and completed the first Gym, less than five minutes after completing the Gym I found a spot moving. Haven't had a problem finding any since.
Just my two cents.
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Best is to stop every so often and listen for it in my experience. The same sound you make when you're walking through the tall grass continues if there is one in the area. Be aware that shaking grass spawns and disapears all the time. If you see a patch of shaking grass and you head for it and a wild pokemon attacks you before you reach it, the shaking grass will be gone after the battle.
Repel is handy to have, just in case one spawns deep into a patch of grass, but in my experience they tend to be near the edges, but that might just be because where I spent time looking for them (outer pinwheel for a Throh) has a lot of edge. But I did test with a repel just to make sure that if you walk into a shaking patch with repel on the battle still activates. It does.
So really my suggestion, and this has worked well for me. Is to dive into the edge of some grass and walk around till you spring a battle, then walk back out onto the path or road or whatever and walk around stopping whenever you have walked far so that all the grass on your screen is new and listen for the rustle. If you hear it look for it, if its silent move on. STAY ON THE SAME ROUTE while doing this, leaving the route of course resets everything. If you can't find a shaky patch, start a random battle and try again. In Black Throh has a 10% spawn rate in shaky grass, and yeah I killed a whole lot of Audinos looking for one, and usually I could pop some tall grass every two non shaky battles or so, sometimes more.