[Kovner]

Yeah - a watch is the way to go. Don't only use your watch to determine whether to bluff on the end or not (in fact, this is probably not a useful application of generating a random number because you couldn't do the TOP calculations to determine Game Theory correct frequency at the table), and you won't be giving off any tells.
For example, Harrington reccomends mixing up how often you raise pre-flop with different hands, what size your bets will be on the flop, etc. Since the many uses overlap, your opponents won't know which reason you're looking at your watch for.
If you're still concerned, you can place your watch on the table near your chips, on your cards, on the arm-rail, etc. so you're just looking down to look at it.
I have one problem with this technique though: If you need to generate a random number twice in one hand (this applies most when the two times are flop, turn or turn, river), the second number is related to the first in a non-random way.
For example, say you determine your continuation bet on the flop should be half the pot because of what your watch says. Your HU opponent instantly calls, the turn comes and you look at your watch again. If this situation happened 100 times, the 2nd number would be different from the 1st by a predictable (on average) amount. Make sense?
Anyways, once you do the watch thing for a couple of sessions you realize its not that neccesary. Mix up your game and keep in mind the general distributions of frequencies for the different situations and you'll be fine.