Since my
first post in this thread, I have chained more than 10 shinies through consecutive fishing. I am now convinced that I can pretty much guarantee I will catch a shiny every time I start a chain, it's just a question of how long it takes.
My shiny chains vary considerably -- I have had chains of length 12, 41, 28, 120, 98, 116, and 35 that I recorded and some others I didn't count specifically. But since I finally figured out how to FISH PROPERLY, I have never had a chain break before catching a shiny.
I could just be getting lucky, but my last several chains (8 or so, iirc) have all ended in a shiny. Every time I have had a chain break, it is because of a lapse in concentration or some other mistake that caused me to pull the line in too early or too late. I'm pretty sure my previously frustrating chain-ends were all because of my own fishing mistakes.
If you are having trouble keeping your chains going, here's what I do to ensure I always "fish correctly".
First off, if you aren't using a Sticky Hold/Suction Cups lead (fainted is fine), then you are going to waste a lot of time. I have found some fishing spots that appear to be very "lucky" in terms of getting long chains without a Sticky/Suction lead, but even the luckiest spots still break chains very, very quickly (no more than 10 or so in a row in good cases) compared to Sticky/Suction chains, which pretty much NEVER break if you are doing it right. With a Sticky/Suction lead, it's almost irrelevant where I fish these days. I can chain for as long as I want on any spot I want and get a "!!" alert on every cast.
Sometimes it takes a very long time for the "!!" to come, but with Sticky/Suction it always comes eventually. In the past, when my chains broke with the maddening "Nothing seems to be biting..." message, I am convinced it is because I had a long pause waiting for the "!!" and the anticipation combined with intense concentration on being sure to press the A button as soon as the "!!" appeared, made me actually press the button slightly and caused the chain to break. I may be wrong, but after adopting a superior fishing technique, I NEVER break chains now when I have a Sticky/Suction lead.
The key to fishing is to press the A button as soon as you HEAR the sound made when the "!!" appears, not when you SEE the "!!" appear. I used to receive many "No! You reeled it in too fast/slow!" messages, before I learned to react to the sound and not the visual. If you are having trouble with it, here's my technique: Don't look at the screen at all. After I cast, I close my eyes or look away from the screen. I wait for the sound and press the A button as soon as I hear it. I use headphones and sit in a quiet room, which makes it even easier to chain consistently. To prevent "squeezing" the DS, which I have found leads to me accidentally pressing the A button -- I set the DS on a table and rest my finger lightly on the A button.
By doing the above, I have caught all the shiny fish-mons I want. Some chains take a while, but they come eventually every time now. Good luck with your fishing!