I used to use some a subtleties which I have no idea if anyone else uses, but anyway...
Even though I don't need to, I classic force the first card so when we find a curious red card in the spread or fan e.t.c. I take it out and look at it and then ask them 'you're card wasn't 'the force card' was it?' (Obviosly I don't ask them if it was the force card

, but the identity of the force card). This suggests nonverbally that the red card's identity really is the card they picked because by taking it directly from the spread I couldn't be doing any false lifts. It is also a nice way to practice the classic force on real people without any real danger- be forceful, but calm with the handling of classic forces, it is all in how you frame it and timing, not a technichal ability.
There then tends to be a little bit of anticipation and tension at which point I put the card back on the packet and there's a small pause for tention and then do a dl and show that the red card really is the card they picked.
I also used to put a lot of emphasis on the red card and that I wasn't swapping it- not too much or it becomes suspicious 'and look that I am not swapping this' is too direct- I simply handled the card very delicatley and said, keep your eye on that. Looking back I'm not sure if this was too overt or not... Another thing I thought would be a nice touch is if to excuse the turning over of the dl- it is after all not the natural thing to do, turn a card back over for no reason-Instead of turning the card over and then turing it back over to get rid of the double, how I tend to handle dls, is by holding them as one card to show the face and the back, applying pressure on either sides as I hold so that the cards don't slip and expose anything, and then almost like a top change with the same misdirection, my hands come together, load the chosen card onto the top and then come back away again which leaves you clean to put it down on the table. I hope that makes sense.
Unfortunatley we didn't get to hear you're presentation, but be creative. If you do gambling routines, then I have heard a very interesting presentation of having a card chosen and then saying 'and to make this card identifiable, we put what is called a crimp in it, so it stand out' and then when the red turns up 'oh.. that seems to have made it stand out a lot', which I think is interesting. This is the thing most worth thinking about, a plot (not necceserily a corny and patronising story- but somehwew to 'place' the magic).
I have gone on for too long, but it is a nice effect, and I hope there is something in my ramblings of value.
edit: I have just been toying with this now and I had a few more ideas. I think it is better to use a dribble force or something in order to put the first chosen card back. The second time when you force a card (Please delete this if this is too much exposure..) you could make it vanish from the deck by any number of means, which I think makes the plot better and more interesting.