By preset I'm thinking of the sort of thing you see in published modules, tightly nailing down the allowable skills and the purposes they can be used for. Which is depressingly common and negating much of the benefit of the challenge mechanics.
OK. Like I said, I don't "preset" the skills or what can be done. In this, I am simply following the advice in the DMG and the PHB (which, as you say, is the only way to make skill challenges' worthwhile). I sometimes will make notes on what will happen if something fairly obvious is tried - preplanning my page 42, if you like.
I regard those published skill challenges as analogous to the "tactics" entry in a combat encounter - advisory, not prescriptive. I think WotC could have done a better job, though, of reconciling its presentation of these with the actual guidelines stated in the rulebooks.
How much information about the mechanics of the challenge did you give the players? Did they know you were running an SC? Did they know what complexity the SC was? Did you set up primary and secondary skills beforehand? Did the players know what DC's they needed to hit to get a success?
They knew I was running a SC. As I described the initial scene, and the players reached for their dice, I expressly asked what they were hoping to achieve.
In the past I have stated the complexity, but I think on this occasion I think I didn't - but I did tell them when they were about halfway through, and then when it was getting to the climax.
I'm not sure what orthodoxy is here, but I treat it a bit like remaining hit points for a monster in a combat encounter - sometimes it suits the mood to say "One more swing will drop it!" and sometimes looser description, and player inference from that as to how things are going, works better.
I didn't predetermine any skills. I had assumed that social skills would be used, and they were. The things that were "innovated" were the couple of occasions when the players engineered for their "weak link" (Derrik, the dwarf) to be physically removed from the situation at least temporarily, and when I let Derrik's player make an Athletics check to correspond to Derrik agreeing with the Baron about being a "man of action".
With DCs, I remember noticing that the players knew they had to hit 20 (for moderate checks). Even at the time I didn't remember having said that, but I must have, because they knew. I certainly told them how much they missed by (if they did) because they had to have the option of spending an AP for +2 or a reroll.
I treat this a bit like monster defences. When the fiction clearly calls for a roll, using a particular skill, and the player is committed, I tend not to worry too much about stating the DC in advance. But when a player is looking for information to help make a call, on a skill check I generally will state the DC (not in combat - but the variation in skill bonuses across a range of options is obviously much wider than is the case with attack bonuses in combat).
Again, I'm not sure how orthodox or unorthodox my DC-stating practice is. I know in BW the obstacle has to be clearly stated - but in BW so much turns on this (advancement, how much artha to spend, etc) and the changes in success chances with each additional step of difficulty are so steep (because it's a dice pool system) that you have to to be fair. I'm not sure 4e is the same in that regard - there are fewer ways than BW to get significant advantages,
and the difficulty gradiant is nowhere near as steep.