I know a few people who have had success with this method.
Get one of those long skinny bags that you get a new fish in from the LFS. (the clear kind) keep the bag over your tank. poke a few holes in the bag, 1 about half way up the bag and then a few holes above that one, nearer to the top of the bag.
Place the bag in the tank and fill with water. Place the nem in the bottom on of the bag (while the bag is still in tank and full of water.) then place the clown in the bag too.
Hold the bag closed with you hand and slowly start to lift the bag out of the water. VERY SLOWLY! This will cause the clown to gently and gradually move toward the nem and touch the nem, without really having another option. Once the clown realizes that it is okay, you can see the clown rubbing on it and touching it in the bag. Leave them like this for a few minutes, until the seem aquainted, then set the nem and clown free.
The people I know who used this said that they found their clowns in the nem the next day, if not the same night.
If you have a nem that is more docile, I don't think it's a huge risk but I wouldn't suggest a forceful union with a condi nem, or like a carpet nem, since they will and do eat fish.