Secondly, the truth is, there aren't that many Dorado to keep you busy whole day. When your flies have enjoyed uneventful bathing for too long, casting lures will give you a welcome diversion. This is not to say the lures catch more dorado than flies. In certain cases, flies in fact produced more actions. But with spinning tackle you can cover by far larger areas at varying depth, collecting wealth of information back. When fishing in a pair, I recommend bringing a set of casting outfit to take turns with.

The rod should be of 6 to 7ft medium heavy action. Spinning reel is also fine when casting light spoons across big waters, but to cast big plugs with accuracy, casting reel works better. We used 30lb test dacron line and a yard of 30lb mono, which is then connected to a wire trace - an absolute must. Because you'd have to tie, untie and cut them many times, nylon-coated wire trace comes handy.
The fierce fight of Dorado is one thing. But the razor sharp teeth of Piranha is another. One gentle touch will make your flies, regardless of their original hairstyle, into the same monk-like bald head. Prepare 10 flies per day, or take your hair-implant surgery kit along.
The combination I found most versatile was 9ft 8wt rod with Type2 sinking tip line. In fast waters where Dorados often hide, Type2 sinking sinks barely deeper than a floating line, but that little extra weight and smaller diameter do make casting bulky flies much easier. Sinking lines are less prone to picking up fine sands on the boat deck or to soften up in the tropical heat, which resulted in easy handling overall.
Another tackle that proved very effective was a solid 10wt, paired up with 450grain sink tip. Now with this heavy line, even the big Dorado flies shoots off. And it does sink well even in a strong current. Casting these heavy lines is understandably tiring, but in certain circumstance, that is simply a must..
Big challenge is how to set the hook on Dorado's rock-hard jaws. We went prepared with good hooks. Even then, chance to land one was only as good as to lose.

A dorado in the hands worths 2 on the hook. And that makes the brief moment of triumph, as you return the gold ore back into the river, so very rewarding.

- Back
Fly Tackle

For surface surface fishing, I brought along a 9ft 7wt rod, with a floating line of 8wt. Okaywhen throwing small flies to dorados hunting minnows in the evening. But it was too light to deliver properly dressed hairy flies especially when short quick cast is required. Such situation occurs very often, not necessarily because you see fish often, but more because you want to hit as many fishy spots from a drifting boat.
Casting Tackle

For two reasons I recommend bringing a casting tackle too.

Firstly, boats are around 20 feet long and not too spacious when two flyfishers are on board. Surely not impossible for two anglers to fish together on calm days, but could become difficult as soon as the wind picks up and the boat starts swirling around. When the boat gets drifted away from the sweet spots you'd have to cast reasonably far, say, around 70ft. Controlling big flies in the air becomes tricky even for a good caster.
Choosing where

Most of the fisheries are within Estancias, privately owned large scale ranches. Staying at these fishing lodges is therefore the only option in most cases. In the Corrientes region where we visited this time, there are several such lodges, including the pioneer Rincon Del Diablo. Most are similarly priced at around US$250/day, with few exceptions such as Pira Lodge and Posada Hambare, which claims to offer another level of luxury - for a price. We spent the first 3days at Estancia la Pelada, near Esquina and the latter 3days at Estancia el Dorado near Mercedes. We found them both equally attractive in two different ways, and honestly, very pleased with the choices we made.

Update 2017) Things have changed. Average prices of guiding & accomodation gone up to US$500 a day.
In the coloured waters flies must have strong attractor features that appeal with sight, sound and commotion. The first choice of colours is black. Black as an universal base, you can put on flashy materials of your choice. Hook size should be 1/0 to 2/0. I even tied huge cannonballs on 4/0 hooks and they received many strikes, but hook up rate was poor. The head of the flies on the other hand must be made real bulky to push the water. Putting on hammer eyes is a good idea to gain that few more inches of crucial depth.
Fishin' Tips