The way it works for me is that I get extremely lucky for very short periods of time but hit long dry spells.
Pre-3.0, I literally pulled up thousands of crawdads trying to get my box. Thanks to me, cooked crawdads actually ended up as one of the cheapest and most abundant foods on the AH for a while. :P I finally got it on the very last wish of my fourth Mr. Pinchy. I then went on to get Old Crafty (not through the daily bug, since my catch predates it) and Old Ironjaw (spent about three days in IF) within the next few days. I happened to get the 1 Ring while I was after Old Ironjaw. So a pretty hot streak after a pretty cold streak, overall.
Post-3.0, aside from Salty, I had almost every fishing achievement that was possible before Wrath launch within a day of the patch going live, either because I had done it already or because I was ready to do it. Only other exceptions were the fishing daily one, since I kept getting unlucky and missing one of them, and the 1k fish one, which I only hit about three days later.
I spent most of my early time in Northrend leveling, but once I set about to get the Northrend fishing achievements, they came pretty easily. I never went after the rat for any significant duration, though, since I didn't need it for Salty, and I would rather spend time fishing in places where I could get a rare fish.
So I've spent about 80% of my time since 3.0 fishing in areas where I could get a rare. My current catch count: 15,184 fish and other things. Being conservative, at least 12k were looking for rares. So even if we assume a 1/5,000 chance of catching a rare, you have a 91% chance of catching one within the number of casts I've made.
On the plus side, with my luck, one of these days, I'll catch like 20 Dark Herring in a row, then go to the Dalaran Sewers and pull up a rat on the first try.
The irony? I caught a rare pre-3.0, but it didn't count since they're all BoE. In fact, it was the rarest one, the Rockhide Strongfish. :P
Edit: Well, sure enough, after 12k casts and over two months of not getting a rare, I get the damn Dark Herring within 30 minutes of making this post. That was the last one I needed for the salty title. Now we'll have to see if I can get the sewer rat today. That's the only fishing achievement I don't have.
